mark multi-form macro expansions with $

A bare list was read as several forms, so a macro returning
`((make-adder 10) 5)' -- a call of what make-adder returns -- was
spliced into two. Add a new $ char to denote splicing, so ($ form ...)
splices.

semen.scm

@@ -52,22 +52,29 @@
       (cons (car forms) (take-until (cdr forms) tail))))
 
 (define (macroexpand macro-form rest-forms)
-  ;; We want to replace macro with its expansion. The problem is,
-  ;; top-level macro can return either a single form, or a list of
-  ;; forms, when it for example generates some aux
-  ;; structures/functions/typedefs.
+  ;; `(defmacro (two) 2)' expands to `2', and `($ (fn a ...) (fn b ...))'
+  ;; to two forms, spliced where the macro was written. `($)' expands to
+  ;; nothing.
   ;;
-  ;; Single form we just cons to the top of rest-forms, but multiple
-  ;; forms have to be appended to the rest-forms.
+  ;; Everything else is one form, a list whose head is itself a form
+  ;; included: `((make-adder 10) 5)' calls what `make-adder' returned,
+  ;; and without `$' to mark a splice there is no telling that from a
+  ;; list of the two forms `(make-adder 10)' and `5'.
   (let ((res (apply-macro macro-form))
         (src (form-source macro-form)))
     ;; An expansion is fresh structure with no location of its own. Give
     ;; it the call site's, the way cpp attributes a macro body to where
     ;; the macro was used
-    (if (list? (car res))
-        (append (map (lambda (f) (stamp-form-source! f src)) res)
-                rest-forms)
-        (cons (stamp-form-source! res src) rest-forms))))
+    (cond
+     ((splice-form? res)
+      (append (map (lambda (f) (stamp-form-source! f src)) (cdr res))
+              rest-forms))
+     ((null? res) rest-forms)
+     (else
+      (cons (stamp-form-source! res src) rest-forms)))))
+
+(define (splice-form? form)
+  (and (pair? form) (list? form) (eq? '$ (car form))))
 
 (define (match-sex-form sex-form acc)
   (match sex-form