Macro rin::math::relative_eq

macro_rules! relative_eq {
    ($lhs:expr, $rhs:expr $(, $opt:ident = $val:expr)*) => { ... };
    ($lhs:expr, $rhs:expr $(, $opt:ident = $val:expr)*,) => { ... };
}

Approximate equality using both the absolute difference and relative based comparisons.