Add a (debug) interactive demo

src/Bergamot/Parser.elm

@@ -26,7 +26,8 @@ variable = Parser.variable
 term : Parser (Term Metavariable)
 term = Parser.lazy (\() -> Parser.oneOf
     [ Parser.succeed IntLit |= intLit
-    , Parser.succeed Call
+    , Parser.backtrackable <|
+      Parser.succeed Call
         |= name
         |= Parser.sequence
             { start = "("
@@ -36,6 +37,8 @@ term = Parser.lazy (\() -> Parser.oneOf
             , item = term
             , trailing = Forbidden
             }
+    , Parser.succeed (\n -> Call n [])
+        |= name
     , Parser.succeed Var |= variable
     ])
 
@@ -70,3 +73,9 @@ program =
         , trailing = Mandatory
         }
     |. Parser.end
+
+run : Parser a -> String -> Maybe a
+run prs s =
+    case Parser.run prs s of
+        Ok a -> Just a
+        Err _ -> Nothing