In module Test§

See primary documentation in context for sub is-approx-calculate

sub is-approx-calculate($got$expected$abs-tol where { !.defined or $_ >= 0 },
                        $rel-tol where { !.defined or $_ >= 0 }$desc)

This is the actual routine called by is-approx when absolute and relative tolerance are specified. They are tested independently, and the test succeeds only if both pass.