approx_equal(${Xmeta ... as str}, "literal") bad semantics

approx_equal(${Xmeta ... as str}, "literal") has rather poor semantics.

See reference.md near l.892, after !302 (merged).

Before 1.0, we must decide whether to change this. It would be a (nominally) breaking change. (Albeit a minor one, so maybe we wouldn't decide to break semver over it.)