Add a message stack to the send list.

def send(self, stack: Layers):
        """
        Add a message stack to the send list.
        """

        if not isinstance(stack, Stack):
            stack = Stack(stack)

        if not self.platform.accept(stack):
            raise UnacceptableStack('The platform does not allow "{}"'
                                    .format(stack.describe()))

        self._stacks.append(stack)