Drain = table(Type,"Drain"){
   function call(key)
      if key in self.m
         return self.m.remove(key)
      else
         return null
      end
   end,
   function index(key)
      return self.m.remove(key)
   end
}

function drain(m)
   return table Drain{m=m}
end

function assert_empty(m)
   if len(m)!=0
      raise table ValueError{
         text = "Value error: unexpected named arguments:\n\s\s{}."%[m]}
   end
end