Can 'sent' HTTP variables work?

I configured:

more_set_headers 'X-Magic: $sent_http_x_slytherin';

Did I understand it correctly that due to module initialisation order, headers-more is executed before the standard headers filter where X-Slytherin is set? Consequently, headers-more will not see X-Slytherin when setting X-Magic.

In other words, the HTTP 'sent' variables as implemented in ngx_http_variable_unknown_header are one of the products of header filters downstream of the headers-more filter that do not yet exist at the time of expanding variables in headers-more.

If I got this right, am I also right that such scenario is an example of undefined behaviour @agentzh referred to in his OpenResty notes when he wrote the below?

Out of sheer curiosity, we shall ask immediately that what would be execution ordering in between more_set_input_headers and rewrite_by_lua, since they both ride on rewrite tail? The answer is : undefined. We must avoid a configuration which relies on their execution orders.

Start at the ngx_http_variable_unknown_header entry point and trace the headers-more filter ordering around more_set_input_headers and rewrite_by_lua. The issue is resolved when the availability and ordering of sent variables are established and the supported behavior or limitation is recorded.