unite :: Sign -> Sign -> Sign
unite sig1 sig2 =
    sig1 { items = Map.union (items sig1) $ items sig2
         }
unite :: Sign -> Sign -> Sign
unite sig1 sig2 =
    sig1 { items = Map.union (items sig1) $ items sig2
         }
unite sig1 sig2 =
    sig1 { items = Map.union (items sig1) $ items sig2
         }