Add SRFI-156 - Syntactic combiners for binary predicates

[jpellegrini]

There was one macro which the reference implementation contains, using syntax-rules, which did not work in STklos, so I write another, using define-macro. All tests pass.

• the problem was that STklos introduces identical symbols in different phases of macro expansion using syntax rules:

(define-syntax f
  (syntax-rules (k)
    ((f) #f)
    ((f k) 'K)
    ((f k x ...) (a (f x ...)))))

Here's the STklos expansion:

stklos> (macro-expand* '(f k k k))
(a (a 'K))

some other implementations will return

(a.0 (a.1 'K))

Anyway, not a problem, since we have define-macro!

[jpellegrini]

stklos> (macro-expand* '(f k k k)) (a (a 'K))

Actually, this seems to affect other SRFIs too. I'll take a look. It seems like, when a macro introduces a new symbol, it should be generated by gensym, and this is not happening...

[jpellegrini]

I have filed issue #55 about this.

[egallesio]

OK. I have sen issue #55. I suppose it will not be simple to correct (I did not wrote this code, and I always have difficulties with it).
I 'll look at the issue asap.
In the meantime, I merge this pool request.