Support "fender" in syntax-rules

[Kon Lovett]

• Allow optional "fender" in `syntax-rules'

• <pattern> <fender> <template>

• <fender> is an <s-expr> that must evaluate to "true"

• <fender> is evaluated as (and <fender> #t)

+ Useful since an er-transformer would otherwise be necessary for very common tasks.

• Muddies syntax-rules in that a procedural element is introduced

---

• (identifier? obj)

(define (identifier? obj)

(let ((sans (strip-syntax obj)))

(and (symbol? sans) (not (keyword? sans)) ) )

-(bound-identifier=? id1 id2)

-(free-identifier=? id1 id2)

[sjamaan]

Personally, I think this doesn't belong in syntax-rules. It's complex enough as it is, and we'll add more features piled on by R7RS soon, too.

This could be an egg.