source: branches/3073_IA_constraint_splitting/HeuristicLab.Problems.DataAnalysis.Symbolic/3.4/Interpreter/IntervalInterpreter.cs @ 17742

#region License Information

/* HeuristicLab
 * Copyright (C) Heuristic and Evolutionary Algorithms Laboratory (HEAL)
 *
 * This file is part of HeuristicLab.
 *
 * HeuristicLab is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * HeuristicLab is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with HeuristicLab. If not, see <http://www.gnu.org/licenses/>.
 */

#endregion

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using HEAL.Attic;
using HeuristicLab.Common;
using HeuristicLab.Core;
using HeuristicLab.Data;
using HeuristicLab.Encodings.SymbolicExpressionTreeEncoding;
using HeuristicLab.Parameters;

namespace HeuristicLab.Problems.DataAnalysis.Symbolic {
  [StorableType("DE6C1E1E-D7C1-4070-847E-63B68562B10C")]
  [Item("IntervalInterpreter", "Interpreter for calculation of intervals of symbolic models.")]
  public sealed class IntervalInterpreter : ParameterizedNamedItem, IStatefulItem {
    private const string EvaluatedSolutionsParameterName = "EvaluatedSolutions";
    private const string MinSplittingWidthParameterName = "MinSplittingWidth";
    private const string MaxSplittingDepthParameterName = "MaxSplittingDepth";
    private const string UseIntervalSplittingParameterName = "UseIntervalSplitting";

    public IFixedValueParameter<IntValue> EvaluatedSolutionsParameter =>
      (IFixedValueParameter<IntValue>) Parameters[EvaluatedSolutionsParameterName];

    public IFixedValueParameter<IntValue> MinSplittingWithParameter =>
      (IFixedValueParameter<IntValue>) Parameters[MinSplittingWidthParameterName];

    public IFixedValueParameter<IntValue> MaxSplittingDepthParameter =>
      (IFixedValueParameter<IntValue>) Parameters[MaxSplittingDepthParameterName];

    public IFixedValueParameter<BoolValue> UseIntervalSplittingParameter =>
      (IFixedValueParameter<BoolValue>) Parameters[UseIntervalSplittingParameterName];

    public int MinSplittingWidth {
      get => MinSplittingWithParameter.Value.Value;
      set => MinSplittingWithParameter.Value.Value = value;
    }

    public int MaxSplittingDepth {
      get => MaxSplittingDepthParameter.Value.Value;
      set => MaxSplittingDepthParameter.Value.Value = value;
    }

    public bool UseIntervalSplitting {
      get => UseIntervalSplittingParameter.Value.Value;
      set => UseIntervalSplittingParameter.Value.Value = value;
    }

    public int EvaluatedSolutions {
      get => EvaluatedSolutionsParameter.Value.Value;
      set => EvaluatedSolutionsParameter.Value.Value = value;
    }

    [StorableConstructor]
    private IntervalInterpreter(StorableConstructorFlag _) : base(_) { }

    private IntervalInterpreter(IntervalInterpreter original, Cloner cloner)
      : base(original, cloner) { }

    public IntervalInterpreter()
      : base("IntervalInterpreter", "Interpreter for calculation of intervals of symbolic models.") {
      Parameters.Add(new FixedValueParameter<IntValue>(EvaluatedSolutionsParameterName,
        "A counter for the total number of solutions the interpreter has evaluated", new IntValue(0)));
      Parameters.Add(new FixedValueParameter<IntValue>(MinSplittingWidthParameterName,
        "Minimum interval width until splitting is stopped", new IntValue(0)));
      Parameters.Add(new FixedValueParameter<IntValue>(MaxSplittingDepthParameterName,
        "Maximum recursion depth of the splitting", new IntValue(5)));
      Parameters.Add(new FixedValueParameter<BoolValue>(UseIntervalSplittingParameterName, "", new BoolValue(false)));
    }

    public override IDeepCloneable Clone(Cloner cloner) {
      return new IntervalInterpreter(this, cloner);
    }

    private readonly object syncRoot = new object();

    #region IStatefulItem Members

    public void InitializeState() {
      EvaluatedSolutions = 0;
    }

    public void ClearState() { }

    #endregion

    public Interval GetSymbolicExpressionTreeInterval(
      ISymbolicExpressionTree tree, IDataset dataset,
      IEnumerable<int> rows = null, int splitDirection = 0) {
      var variableRanges = DatasetUtil.GetVariableRanges(dataset, rows);
      return GetSymbolicExpressionTreeInterval(tree, variableRanges);
    }

    public Interval GetSymbolicExpressionTreeIntervals(
      ISymbolicExpressionTree tree, IDataset dataset,
      out IDictionary<ISymbolicExpressionTreeNode, Interval>
        nodeIntervals, IEnumerable<int> rows = null, int splitDirection = 0) {
      var variableRanges = DatasetUtil.GetVariableRanges(dataset, rows);
      return GetSymbolicExpressionTreeIntervals(tree, variableRanges, out nodeIntervals);
    }

    public Interval GetSymbolicExpressionTreeInterval(
      ISymbolicExpressionTree tree,
      IReadOnlyDictionary<string, Interval> variableRanges, int splitDirection = 0) {
      lock (syncRoot) {
        EvaluatedSolutions++;
      }

      Interval outputInterval;

      if (UseIntervalSplitting) {
        outputInterval = GetSymbolicExpressionTreeIntervals(tree, variableRanges,
          out var nodeIntervals);
      } else {
        var instructionCount = 0;
        var instructions = PrepareInterpreterState(tree, variableRanges);
        outputInterval = Evaluate(instructions, ref instructionCount);
      }

      return outputInterval.LowerBound <= outputInterval.UpperBound
        ? outputInterval
        : new Interval(outputInterval.UpperBound, outputInterval.LowerBound);
    }


    public Interval GetSymbolicExpressionTreeIntervals(
      ISymbolicExpressionTree tree,
      IReadOnlyDictionary<string, Interval> variableRanges,
      out IDictionary<ISymbolicExpressionTreeNode, Interval>
        nodeIntervals, int splitDirection = 0) {
      lock (syncRoot) {
        EvaluatedSolutions++;
      }

      var intervals = new Dictionary<ISymbolicExpressionTreeNode, Interval>();
      var instructions = PrepareInterpreterState(tree, variableRanges);

      Interval outputInterval;
      if (UseIntervalSplitting) {
        var variables = tree.IterateNodesPrefix().OfType<VariableTreeNode>().Select(x => x.VariableName).Distinct()
                            .ToList();
        var containsDependencyProblem = ContainsVariableMultipleTimes(tree);

        if (containsDependencyProblem) {
          var currIndex = 0;
          var currDepth = 0;
          IDictionary<string, Interval> writeableVariableRanges =
            variableRanges.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
          //outputInterval = EvaluateRecursive(instructions, intervals, writeableVariableRanges, variables, MinSplittingWidth, MaxSplittingDepth,
          //  ref currIndex, ref currDepth, tree);
          outputInterval = EvaluateWithSplitting(instructions, intervals, writeableVariableRanges, splitDirection);
        } else {
          var instructionCount = 0;
          outputInterval = Evaluate(instructions, ref instructionCount, intervals);
        }
      } else {
        var instructionCount = 0;
        outputInterval = Evaluate(instructions, ref instructionCount, intervals);
      }

      nodeIntervals = new Dictionary<ISymbolicExpressionTreeNode, Interval>();
      foreach (var kvp in intervals) {
        var interval = kvp.Value;
        if (interval.IsInfiniteOrUndefined || interval.LowerBound <= interval.UpperBound)
          nodeIntervals.Add(kvp.Key, interval);
        else
          nodeIntervals.Add(kvp.Key, new Interval(interval.UpperBound, interval.LowerBound));
      }

      // because of numerical errors the bounds might be incorrect
      if (outputInterval.IsInfiniteOrUndefined || outputInterval.LowerBound <= outputInterval.UpperBound)
        return outputInterval;

      return new Interval(outputInterval.UpperBound, outputInterval.LowerBound);
    }


    private static Instruction[] PrepareInterpreterState(
      ISymbolicExpressionTree tree,
      IReadOnlyDictionary<string, Interval> variableRanges) {
      if (variableRanges == null)
        throw new ArgumentNullException("No variablew ranges are present!", nameof(variableRanges));

      //Check if all variables used in the tree are present in the dataset
      foreach (var variable in tree.IterateNodesPrefix().OfType<VariableTreeNode>().Select(n => n.VariableName)
                                   .Distinct())
        if (!variableRanges.ContainsKey(variable))
          throw new InvalidOperationException($"No ranges for variable {variable} is present");

      var code = SymbolicExpressionTreeCompiler.Compile(tree, OpCodes.MapSymbolToOpCode);
      foreach (var instr in code.Where(i => i.opCode == OpCodes.Variable)) {
        var variableTreeNode = (VariableTreeNode) instr.dynamicNode;
        instr.data = variableRanges[variableTreeNode.VariableName];
      }

      return code;
    }

    public static Interval EvaluateWithSplitting(Instruction[] instructions,
                                                 IDictionary<ISymbolicExpressionTreeNode, Interval> nodeIntervals,
                                                 IDictionary<string, Interval> variableIntervals,
                                                 int splitDirection = 0) {
      if (splitDirection == 0) {
        var savedIntervals = variableIntervals.ToDictionary(entry => entry.Key, entry => entry.Value);
        var minimization = PerformSplitting(instructions, nodeIntervals, variableIntervals, -1);
        var maximization = PerformSplitting(instructions, nodeIntervals, savedIntervals, 1);

        return new Interval(minimization.LowerBound, maximization.UpperBound);
      }

      return PerformSplitting(instructions, nodeIntervals, variableIntervals, splitDirection);
    }

    private static Interval PerformSplitting(Instruction[] instructions,
                                             IDictionary<ISymbolicExpressionTreeNode, Interval> nodeIntervals,
                                             IDictionary<string, Interval> variableIntervals, int splitDirection) {
      SortedList<Tuple<double, List<Interval>>, Interval> priorityList = null;
      priorityList = splitDirection == -1 ? new SortedList<Tuple<double, List<Interval>>, Interval>(new BoxMinimizer()) : new SortedList<Tuple<double, List<Interval>>, Interval>(new BoxMaximizer());
      var ic = 0;
      //Calculate full box
      IReadOnlyDictionary<string, Interval> readonlyRanges = variableIntervals.ToDictionary(k => k.Key, k => k.Value);
      var fullbox = Evaluate(instructions, ref ic, nodeIntervals, readonlyRanges);
      var box = variableIntervals.Keys.Select(k => variableIntervals[k]).ToList();
      priorityList.Add(
        splitDirection == -1
          ? new Tuple<double, List<Interval>>(fullbox.LowerBound, box)
          : new Tuple<double, List<Interval>>(fullbox.UpperBound, box), fullbox);

      for (var depth = 0; depth < 200; ++depth) {
        var firstBox = priorityList.First();
        priorityList.RemoveAt(0);

        var boxes = Split(firstBox.Key.Item2);

        foreach (var b in boxes) {
          var i = 0;
          foreach (var k in readonlyRanges.Keys) {
            variableIntervals[k] = b.ToList()[i];
            ++i;
          }

          ic = 0;
          var res = Evaluate(instructions, ref ic, nodeIntervals,
            new ReadOnlyDictionary<string, Interval>(variableIntervals));
          priorityList.Add(
            splitDirection == -1
              ? new Tuple<double, List<Interval>>(res.LowerBound, b.ToList())
              : new Tuple<double, List<Interval>>(res.UpperBound, b.ToList()), res);
        }
      }

      //Calculate final interval
      var final = new Interval(0, 0);

      return priorityList.Aggregate(final, (current, b) => current | b.Value);
    }

    private static IEnumerable<IEnumerable<Interval>> Split(List<Interval> box) {
      var boxes = box.Select(region => region.Split()).Select(split => new List<Interval> {split.Item1, split.Item2})
                     .ToList();

      return boxes.CartesianProduct();
    }

    private class BoxMinimizer : IComparer<Tuple<double, List<Interval>>> {
      public int Compare(Tuple<double, List<Interval>> x, Tuple<double, List<Interval>> y) {
        var result = x.Item1.CompareTo(y.Item1);

        if (result == 0) {
          var sizeBox2 = y.Item2.Aggregate(1.0, (current, region) => current * region.Width);
          var sizeBox1 = x.Item2.Aggregate(1.0, (current, region) => current * region.Width);

          if (sizeBox2 < sizeBox1) return -1;

          return 1;
        }

        return result;
      }
    }

    private class BoxMaximizer : IComparer<Tuple<double, List<Interval>>> {
      public int Compare(Tuple<double, List<Interval>> x, Tuple<double, List<Interval>> y) {
        var result = y.Item1.CompareTo(x.Item1);

        if (result == 0) {
          var sizeBox2 = y.Item2.Aggregate(1.0, (current, region) => current * region.Width);
          var sizeBox1 = x.Item2.Aggregate(1.0, (current, region) => current * region.Width);

          if (sizeBox2 > sizeBox1) return -1;

          return 1;
        }

        return result;
      }
    }

    public static Interval EvaluateRecursive(
      Instruction[] instructions,
      IDictionary<ISymbolicExpressionTreeNode, Interval> nodeIntervals,
      IDictionary<string, Interval> variableIntervals, IList<string> variables,
      double minWidth, int maxDepth, ref int currIndex, ref int currDepth,
      ISymbolicExpressionTree tree) {
      Interval evaluate() {
        var ic = 0;
        IReadOnlyDictionary<string, Interval> readonlyRanges =
          new ReadOnlyDictionary<string, Interval>(variableIntervals);
        return Evaluate(instructions, ref ic, nodeIntervals, readonlyRanges);
      }

      Interval recurse(ref int idx, ref int depth) {
        return EvaluateRecursive(instructions, nodeIntervals, variableIntervals, variables, minWidth, maxDepth, ref idx,
          ref depth, tree);
      }


      var v = variables[currIndex];
      var x = variableIntervals[v];
      if (x.Width < minWidth || currDepth == maxDepth || !MultipleTimes(tree, v)) {
        if (currIndex + 1 < variables.Count) {
          currDepth = 0;
          currIndex++;
          var z = recurse(ref currIndex, ref currDepth);
          currIndex--;
          return z;
        }

        return evaluate();
      }

      var t = x.Split();
      var xa = t.Item1;
      var xb = t.Item2;
      var d = currDepth;
      currDepth = d + 1;
      variableIntervals[v] = xa;
      var ya = recurse(ref currIndex, ref currDepth);
      currDepth = d + 1;
      variableIntervals[v] = xb;
      var yb = recurse(ref currIndex, ref currDepth);
      variableIntervals[v] = x; // restore interval
      return ya | yb;
    }

    public static Interval Evaluate(
      Instruction[] instructions, ref int instructionCounter,
      IDictionary<ISymbolicExpressionTreeNode, Interval> nodeIntervals = null,
      IReadOnlyDictionary<string, Interval> variableIntervals = null) {
      var currentInstr = instructions[instructionCounter];
      //Use ref parameter, because the tree will be iterated through recursively from the left-side branch to the right side
      //Update instructionCounter, whenever Evaluate is called
      instructionCounter++;
      Interval result = null;

      switch (currentInstr.opCode) {
        //Variables, Constants, ...
        case OpCodes.Variable: {
          var variableTreeNode = (VariableTreeNode) currentInstr.dynamicNode;
          var weightInterval = new Interval(variableTreeNode.Weight, variableTreeNode.Weight);
          //var variableInterval = (Interval)currentInstr.data;

          Interval variableInterval;
          if (variableIntervals != null && variableIntervals.ContainsKey(variableTreeNode.VariableName))
            variableInterval = variableIntervals[variableTreeNode.VariableName];
          else
            variableInterval = (Interval) currentInstr.data;

          result = Interval.Multiply(variableInterval, weightInterval);
          break;
        }
        case OpCodes.Constant: {
          var constTreeNode = (ConstantTreeNode) currentInstr.dynamicNode;
          result = new Interval(constTreeNode.Value, constTreeNode.Value);
          break;
        }
        //Elementary arithmetic rules
        case OpCodes.Add: {
          //result = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          result = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          for (var i = 1; i < currentInstr.nArguments; i++) {
            //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
            var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
            result = Interval.Add(result, argumentInterval);
          }

          break;
        }
        case OpCodes.Sub: {
          //result = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          result = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          if (currentInstr.nArguments == 1)
            result = Interval.Multiply(new Interval(-1, -1), result);

          for (var i = 1; i < currentInstr.nArguments; i++) {
            //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
            var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
            result = Interval.Subtract(result, argumentInterval);
          }

          break;
        }
        case OpCodes.Mul: {
          //result = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          result = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          for (var i = 1; i < currentInstr.nArguments; i++) {
            var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
            //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
            result = Interval.Multiply(result, argumentInterval);
          }

          break;
        }
        case OpCodes.Div: {
          //result = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          result = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          if (currentInstr.nArguments == 1)
            result = Interval.Divide(new Interval(1, 1), result);

          for (var i = 1; i < currentInstr.nArguments; i++) {
            //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
            var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
            result = Interval.Divide(result, argumentInterval);
          }

          break;
        }
        //Trigonometric functions
        case OpCodes.Sin: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.Sine(argumentInterval);
          break;
        }
        case OpCodes.Cos: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.Cosine(argumentInterval);
          break;
        }
        case OpCodes.Tan: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.Tangens(argumentInterval);
          break;
        }
        case OpCodes.Tanh: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.HyperbolicTangent(argumentInterval);
          break;
        }
        //Exponential functions
        case OpCodes.Log: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.Logarithm(argumentInterval);
          break;
        }
        case OpCodes.Exp: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.Exponential(argumentInterval);
          break;
        }
        case OpCodes.Square: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.Square(argumentInterval);
          break;
        }
        case OpCodes.SquareRoot: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.SquareRoot(argumentInterval);
          break;
        }
        case OpCodes.Cube: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.Cube(argumentInterval);
          break;
        }
        case OpCodes.CubeRoot: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.CubicRoot(argumentInterval);
          break;
        }
        case OpCodes.Absolute: {
          //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          result = Interval.Absolute(argumentInterval);
          break;
        }
        case OpCodes.AnalyticQuotient: {
          //result = Evaluate(instructions, ref instructionCounter, nodeIntervals);
          result = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
          for (var i = 1; i < currentInstr.nArguments; i++) {
            //var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals);
            var argumentInterval = Evaluate(instructions, ref instructionCounter, nodeIntervals, variableIntervals);
            result = Interval.AnalyticalQuotient(result, argumentInterval);
          }

          break;
        }
        default:
          throw new NotSupportedException($"The tree contains the unknown symbol {currentInstr.dynamicNode.Symbol}");
      }

      if (!(nodeIntervals == null || nodeIntervals.ContainsKey(currentInstr.dynamicNode)))
        nodeIntervals.Add(currentInstr.dynamicNode, result);

      return result;
    }

    private static bool MultipleTimes(ISymbolicExpressionTree tree, string variable) {
      var varlist = tree.IterateNodesPrefix().OfType<VariableTreeNode>().GroupBy(x => x.VariableName);
      var group = varlist.Select(x => x.Key == variable).Count();

      return group > 1;
    }

    private static bool ContainsVariableMultipleTimes(ISymbolicExpressionTree tree) {
      var varlist = tree.IterateNodesPrefix().OfType<VariableTreeNode>().GroupBy(x => x.VariableName);
      return varlist.Any(group => group.Count() > 1);
    }


    public static bool IsCompatible(ISymbolicExpressionTree tree) {
      var containsUnknownSymbols = (
        from n in tree.Root.GetSubtree(0).IterateNodesPrefix()
        where
          !(n.Symbol is Variable) &&
          !(n.Symbol is Constant) &&
          !(n.Symbol is StartSymbol) &&
          !(n.Symbol is Addition) &&
          !(n.Symbol is Subtraction) &&
          !(n.Symbol is Multiplication) &&
          !(n.Symbol is Division) &&
          !(n.Symbol is Sine) &&
          !(n.Symbol is Cosine) &&
          !(n.Symbol is Tangent) &&
          !(n.Symbol is HyperbolicTangent) &&
          !(n.Symbol is Logarithm) &&
          !(n.Symbol is Exponential) &&
          !(n.Symbol is Square) &&
          !(n.Symbol is SquareRoot) &&
          !(n.Symbol is Cube) &&
          !(n.Symbol is CubeRoot) &&
          !(n.Symbol is Absolute) &&
          !(n.Symbol is AnalyticQuotient)
        select n).Any();
      return !containsUnknownSymbols;
    }
  }
}