fun test(reg, x, y, reg') =
      let val reg  = Reg.fromString reg
          val x    = Str.fromString x
          val y    = Str.fromString y
          val reg' = Reg.fromString reg'

          val fa  = subst(regToFA reg, x, y)
          val fa' = regToFA reg'
      in DFA.equivalent(faToDFA fa, faToDFA fa') end;