-- mget! — one-step Optional unwrap with nil propagation.
--
-- `mget m k` returns `O v` (nil if the key is missing, the value if present).
-- Before mget! existed, every reader had to write a two-step bind:
-- r = mget m "k"
-- v = r ?? 0
-- to extract a value, even when the key was known to be present.
--
-- mget! collapses that to a single call: if the key is present, the call
-- yields the value; if missing, nil propagates out of the enclosing function
-- (which therefore must return an Optional).
--
-- Parallels the existing `!` on R-returning calls: Ok→v, Err→propagate.
-- Present key: mget! yields the inner value (5).
present>O n;m=mset mmap "k" 5;v=mget! m "k";v
-- Missing key: mget! propagates nil immediately. The `+v 99` is never reached.
missing>O n;m=mmap;v=mget! m "k";+v 99
-- The two-step `??` idiom still works when a default is preferred over
-- propagation.
defaulted>n;m=mmap;r=mget m "k";v=r??42;v
-- run: present
-- out: 5
-- run: missing
-- out: nil
-- run: defaulted
-- out: 42