-- | Read a rule.
parseRule :: [String] -> TransMap -> TransMap
parseRule [src, "=>", dst] m = insertMulti [src] es m
  where
    es = envelop parseElem [] dst
parseRule :: [String] -> TransMap -> TransMap
parseRule [src, "=>", dst] m = insertMulti [src] es m
  where
    es = envelop parseElem [] dst
parseRule [src, "=>", dst] m = insertMulti [src] es m
  where
    es = envelop parseElem [] dst