One TRUE or FALSE. Branching on a flag with isTRUE() alone accepts the
value silently but only ever takes its "false" branch for anything that
is not the literal value TRUE – "yes", 1, or "TRUE" as a string all
pass through and flip what the caller gets back with no error (#321).
check_flag() rejects those up front so the mistake is caught at the call
that made it rather than read off a wrong-shaped result.