semigroup-monoid.ls

# semigroup
Readable::concat = (b)->
  a = this

  new class extends Readable
    ->
      super ...
      @read-from = a

    _read: (size)->
      if (@read-from.read size)?
        @push that
      else if @read-from is a
        @read-from = b
        @push ""
      else
        @push null


# monoid
Readable.empty = -> Readable.of ""