Postgres: modify existing search_path (preserve current values)

Sometimes I would like to extend the existing search path, rather than replace it, say:

To start with say it is already set like so:

SET search_path TO schema_b, schema_c, public;

I want to add my schema to the front of the line:

SET search_path TO schema_a + search_path;   --doesn't work

I’m thinking in analogy to what I’d do in BASH:

PATH=path_a:$PATH

Bonus question, perhaps related: is there a way I can store the current path temporarily, so I can change it to something totally different, and then restore it without having to know what it was?