Function notation for Prolog

This is simple term_expansion/2 based macros for functions notation.

This is a simple transform, the output is still easy to debug, it doesn't require special notation for function's definitions, any predicate may be used as a funciton already.

The simpliest usage is for predicate with the only output argument in the last position. But any predicate with any position of output arguments and any number of them may be called as a function. In case of more than one arguments only one can be specified by placeholder but the others may still be bound with some named variable.

Use $> and $< operators in subterms of head or body to specify function's term and its output variable placeholder respectively. If the variable placeholder isn't specified the library will add another (last) argument for the term and use it as an output variable.

e.g.

h1(A, $> h2()) :- p1(A, $> p2($> p3($<, A))) .

and

h1(A, $> h2($<)) :- p1(A, $> p2($> p3($<, A), $<)) .

is converted into:

h1(A,V1) :- h2(V1), p3(V0, A), p2(V0, V2), p1(A, V2) .

The operators names may be changed, for example making both of them to be $$:

:- fnotation_ops($$,$$) .
:- op(900,fx,$$) .

h1(A,R) :- p1($$ p2($$, A), R) .