The '''Probabilistic Tree Creator''' tries to create symbolic expression trees with uniformly distributed length.

'''Initial phase''': create a root node and a start node (which together amount for an initial 
tree length of 1), and begin expanding until the target length is reached. This works by 
initially placing dummy extension points onto the available children placeholders, then 
replacing them with subtrees of specific depth and length. 

The number of children is determined 
in a more sophisticated way: the minimum/maximum arities are calculated based on the cumulative 
max/min expression lengths of the allowed child symbols so that the number of nodes will fit the 
desired tree length.

 [[Image(PTC2-01.png, 450px)]] 

'''Next phase''': pick random extension points, replace them with subtrees (of a minimal 
or otherwise constrained size), depending on the current depth and length of the tree.

 [[Image(PTC2-02.png, 450px)]] 

The subtrees will have their own extension points, and so on; the algorithm iterates 
over the remaining extension points and fills them up while keeping count of the 
current tree length and depth.