"rank" simply does not work nor learns anything...

Randomize programs in a fixed way: values close to x.5 get cast to int(x), values further away may slip to x-1 or x+1

Constant propagation does not resolve things like (x[0] != x[0]) as
False, which then prevents the evaluation from detecting that the
program is not actually looking at x. By removing any operator that may
map a function of x to a constant, we make sure that a program that has
"x" in its expression actually has an x-dependent output.

NOTE: This commit does not address a corner case, of exp(large-number)
being a constant (exp(10)). Maybe some programs may learn to use exp and
then a division by a constant to fabricate a constant even though "x"
appears in their string representation

Add negative versions of functions that have a positive image, so, on
average on random DNAs, the output of a program is not biased towards
positive numbers.