Function mixed_numeral_fraction 

pub fn mixed_numeral_fraction(
    numerator: &BigInt,
    denominator: &BigInt,
) -> Result<String>

mixed_numeral for an exact fraction, whose denominator is not limited: -4 3/7 for -31/7.

use music21::common::numbertools::mixed_numeral_fraction;

assert_eq!(mixed_numeral_fraction(&31.into(), &7.into())?, "4 3/7");
assert_eq!(mixed_numeral_fraction(&(-4).into(), &5.into())?, "-4/5");

Errors

A denominator of nought, or a fraction too large for a float, which music21 passes through on the way.