After this commit, there is no need to provide an explicit mapping, since it is assumed that the propositions *are* the ground fluents.

E.g. if previously the formula was specified as follows:

formula = "on_b_a & O(ontable_c)"

Now we require that the propositions can be interpreted as PDDL predicates, i.e.:

formula = '"on b a" & O("ontable c")'

Pylogics does not allow spaces in the proposition name; therefore, the double quotes are always required.

Only exception is when the predicate is unary:

formula = 'Y(O(made-p4)))'

(as in openstacks).

We now check if the prefix 'val' is in the atom name. This is because after the compilation there might be conflicts in the val predicate names.

Consider the following example: '"p a b" & val-p-a-b'

The formula is compiled as follows:

"p a b" ->  'val-p-a-b'

(:derived (val-p-a-b)        (p a b))
	      ^                  ^
	      derived-predicate  condition (ground fluent, predicate name 'p', constants 'a' and 'b')

"val-p-a-b" -> 'val-val-p-a-b'

(:derived (val-val-p-a-b)     (val-p-a-b))
	      ^                   ^
	      derived-predicate   condition (ground fluent, predicate name 'p-a-b', no constants

The prefix is changed to `val__` to make the prefix less probable to occur in an atom name.

with this change, we translate each formula special symbols (operator symbols, parenthesis etc.) into a sequence of characters that can be part of a valid derived predicate name.

This means that, given a name of a derived predicate, one can simply do the inverse replacement and recover the original formula.

This approach has two major drawbacks:
1) the output of the compiler is much more verbose; e.g. the formula 'Y(a)' gets translated into 'val__YLPAR__a__RPAR'
2) the new approach imposes restrictions on the allowed symbols in a PPLTL formula. E.g. symbols that start with 'VAL__' or 'Y__', or symbols that contain one of the following as substring:
  - "LPAR__" (left parenthesis)
  - "__RPAR" (right parenthesis)
  - "NOT__" (not symbol)
  - "__AND__" (and symbol)
  - "__OR__" (or symbol)
  - "__QUOTE__" (quote)
  are forbidden.

This policy might be more conservative than needed, but in this way we should have the guarantee that the encoding is an invertible mapping (such property basically proves non-ambiguity of the encoding, because it means there is a one-to-one relationship between 'string representation of pylogics formulas' and 'string representation encoded as PDDL predicate name').