* New language feature: domain checks, which check whether a function

argument has a valid value, i.e., is in a certain domain.  E.g.,

    { foo : [true false]
    , bar : ["a" "b" "c"]
    }: ...

  This previously could be done using assertions, but domain checks
  will allow the buildfarm to automatically extract the configuration
  space from functions.

src/libexpr/parser.y

@@ -206,7 +206,7 @@ formals
 
 formal
   : ID { $$ = makeFormal($1, makeUnrestrictedValues(), makeNoDefaultValue()); }
-//  | ID ':' '[' expr_list ']' { $$ = makeDefFormal($1, $3); }
+  | ID ':' '[' expr_list ']' { $$ = makeFormal($1, makeValidValues($4), makeNoDefaultValue()); }
   | ID '?' expr { $$ = makeFormal($1, makeUnrestrictedValues(), makeDefaultValue($3)); }
   ;