Added a new wait guard (that matches the Rain wait guards exactly) to the AST

common/AST.hs

@@ -335,6 +335,8 @@ data Alternative =
   -- | A @SKIP@ guard (one that is always ready).
   -- The 'Expression' is the pre-condition.
   | AlternativeSkip Meta Expression Process
+  -- | A @wait for@/@until@ guard.
+  | AlternativeWait Meta WaitMode Expression Process
   deriving (Show, Eq, Typeable, Data)
 
 -- | An option in a @CASE@ process.