Start to implementation of syntax matcher for requirements.

So far, I'm just recording a few ideas about the implementation.
There's a long way to go.

codeworld-compiler/codeworld-compiler.cabal

@@ -37,6 +37,7 @@ Library
     CodeWorld.Compile.Requirements.Eval
     CodeWorld.Compile.Requirements.Language
     CodeWorld.Compile.Requirements.LegacyLanguage
+    CodeWorld.Compile.Requirements.Matcher
     CodeWorld.Compile.Requirements.Types
     CodeWorld.Compile.Stages
 

codeworld-compiler/src/CodeWorld/Compile/Requirements/Matcher.hs

@@ -0,0 +1,30 @@
+{-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE ViewPatterns #-}
+
+{-
+  Copyright 2018 The CodeWorld Authors. All rights reserved.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+-}
+
+module CodeWorld.Compile.Requirements.Matcher where
+
+import Data.List
+import Language.Haskell.Exts
+
+pattern SpecialExp nm <- Var _ (UnQual _ (Ident _ (specialName -> Just nm)))
+pattern SpecialPat nm <- PVar _ (Ident _ (specialName -> Just nm))
+
+specialName :: String -> Maybe String
+specialName s | "__" `isPrefixOf` s = Just s
+              | otherwise           = Nothing