Add a new function to get the parents of a symbol in a grammar

Author: sungshik

rascal-textmate-core/src/main/rascal/lang/rascal/grammar/Util.rsc

@@ -108,6 +108,17 @@ list[Pointer] find(Grammar g, Production p, Symbol s, Direction dir = forward())
     return doFind({}, p, s);
 }
 
+@synopsis{
+    Lookdowns a list of productions for symbol `s` in grammar `g`
+}
+
+// TODO: Rename this function because the current name makes little sense in
+// isolation (it's supposed to be the opposite of `lookup`, but in that sense,
+// the directions are illogical)
+
+set[Production] lookdown(Grammar g, Symbol s)
+    = {parent | /parent: prod(_, /Symbol _: s, _) := g};
+
 @synopsis{
     Lookups a list of productions for symbol `s` in grammar `g`, replacing
     formal parameters with actual parameters when needed