Improves the error reporting and documentation for `define-relation'

collects/redex/private/reduction-semantics.rkt

@@ -1462,7 +1462,7 @@
                     (when (null? (cdr more))
                       (raise-syntax-error syn-error-name 
                                           (format "expected a pattern to follow ~a" (syntax-e (car more)))
-                                          stx))
+                                          stx (car more)))
                     (loop (cddr more)
                           (cons (cadr more) arg-pats))]
                    [else
@@ -1490,7 +1490,8 @@
          [_
           (raise-syntax-error
            syn-error-name
-           "expected the name of the meta-function, followed by its contract (or no name and no contract)"
+           (format "expected the name of the ~a, followed by its contract (or no name and no contract)"
+                   (if relation? "relation" "meta-function"))
            stx
            rest)])]))
            

collects/redex/redex.scrbl

@@ -1042,6 +1042,7 @@ and @racket[#f] otherwise.
 
 @defform/subs[#:literals ()
               (define-relation language
+               relation-contract
                [(name @#,ttpattern ...) @#,tttterm ...] ...)
                ([relation-contract (code:line)
                                    (code:line id ⊂ pat x ... x pat)

collects/redex/tests/tl-test.rkt

@@ -929,7 +929,10 @@
             'failed)
           'passed))
   
- 
+  (test-syn-err 
+   (define-relation grammar R)
+   #rx"expected the name of the relation")
+  
 ;                    ;;                         ;                                        ;;                    ;                 
 ;                     ;                 ;                                                 ;            ;                         
 ;   ;; ;;   ;;;    ;; ; ;;  ;;   ;;;;  ;;;;;  ;;;     ;;;  ;; ;;          ;; ;;   ;;;     ;     ;;;   ;;;;;  ;;;     ;;;  ;; ;;