def _stack_from_spec(self, spec, stack=(), seen_names=()):
        """
        Return a tuple of ContextService parameter dictionaries corresponding
        to the connection described by `spec`, and any connection referenced by
        its `mitogen_via` or `become` fields. Each element is a dict of the
        form::

            {
                # Optional. If present and `True`, this hop is elegible for
                # interpreter recycling.
                "enable_lru": True,
                # mitogen.master.Router method name.
                "method": "ssh",
                # mitogen.master.Router method kwargs.
                "kwargs": {
                    "hostname": "..."
                }
            }

        :param ansible_mitogen.transport_config.Spec spec:
            Connection specification.
        :param tuple stack:
            Stack elements from parent call (used for recursion).
        :param tuple seen_names:
            Inventory hostnames from parent call (cycle detection).
        :returns:
            Tuple `(stack, seen_names)`.
        """
        if spec.inventory_name() in seen_names:
            raise ansible.errors.AnsibleConnectionFailure(
                self.via_cycle_msg % (
                    spec.mitogen_via(),
                    spec.inventory_name(),
                    ' -> '.join(reversed(
                        seen_names + (spec.inventory_name(),)
                    )),
                )
            )

        if spec.mitogen_via():
            stack = self._stack_from_spec(
                self._spec_from_via(spec.inventory_name(), spec.mitogen_via()),
                stack=stack,
                seen_names=seen_names + (spec.inventory_name(),),
            )

        stack += (CONNECTION_METHOD[spec.transport()](spec),)
        if spec.become() and ((spec.become_user() != spec.remote_user()) or
                              C.BECOME_ALLOW_SAME_USER):
            stack += (CONNECTION_METHOD[spec.become_method()](spec),)

        return stack
Return a tuple of ContextService parameter dictionaries corresponding
        to the connection described by `spec`, and any connection referenced by
        its `mitogen_via` or `become` fields. Each element is a dict of the
        form::

            {
                # Optional. If present and `True`, this hop is elegible for
                # interpreter recycling.
                "enable_lru": True,
                # mitogen.master.Router method name.
                "method": "ssh",
                # mitogen.master.Router method kwargs.
                "kwargs": {
                    "hostname": "..."
                }
            }

        :param ansible_mitogen.transport_config.Spec spec:
            Connection specification.
        :param tuple stack:
            Stack elements from parent call (used for recursion).
        :param tuple seen_names:
            Inventory hostnames from parent call (cycle detection).
        :returns:
            Tuple `(stack, seen_names)`.