Macro span

macro_rules! span {
    ($n1:literal/$d1:literal, $n2:literal/$d2:literal) => { ... };
    ($n1:literal/$d1:literal, $r2:expr) => { ... };
    ($r1:expr, $n2:literal/$d2:literal) => { ... };
    ($r1:expr, $r2:expr) => { ... };
    ($n:literal / $d:literal) => { ... };
    ($r:expr) => { ... };
}

A shorthand macro for constructing spans from rationals, e.g. span!(0/1, 3/1).