The parser for symbolic expressions in infix form parses negative numbers and fractions strangly

[gkronber]

Some examples:

• Input: -1.0*x + 2.0
• Expected:

    (+ 
       (* -1.0 x) 
       2.0)
  

• Output:

  (+ 
    (- (* 1.0 x))
    2.0)
  

• Input: exp(-1.0*x - 2.0)
• Output:

  (exp (* 
         -1.0 
         (+ 
           (* 1.0 x) 
           2.0)))
  

• Expected:

  (exp (- 
         (* -1.0 x)
          2.0))
  

• Input: 3/3+2/2+1/1
• Output:

   ((3 * 1/3) + (2 * 1/2) + (1 * 1/1))
  

[gkronber]

The main problem is that by parsing "-x" as (-1)*x we effectively introduce an additional constant (see non-linear regression). The same issue occurs when parsing "x/y" as "x * 1/y".

Parsing "-3.14" as "(-1)*3.14" is especially strange.

[gkronber]

r16357: added comments for infix parser tests