step: add some q-expression functions, part 1 of many.

src/mumble.c

@@ -377,7 +377,8 @@ int main(int argc, char** argv)
     /* Define them with the following Language */
     mpca_lang(MPCA_LANG_DEFAULT, "                           \
     number   : /-?[0-9]+\\.?([0-9]+)?/ ;                     \
-    symbol : '+' | '-' | '*' | '/' ;                         \
+    symbol : \"list\" \"head\" \"tail\" \"eval\"             \
+		       | '+' | '-' | '*' | '/' ;                         \
 		sexpr : '(' <expr>* ')' ;                                \
 		qexpr : '{' <expr>* '}' ;                                \
     expr     : <number> | <symbol> | <sexpr> | <qexpr>;      \