Function unique_name_of_plural 

pub fn unique_name_of_plural(plural: &str) -> Option<&'static str>

The unique name a plural attribute of music21’s answers for, composer for composers: music21’s ALL_PLURAL_ATTRIBUTE_NAMES, each of which answers every value under the name rather than the first.

use music21::metadata::unique_name_of_plural;

assert_eq!(unique_name_of_plural("composers"), Some("composer"));
assert_eq!(unique_name_of_plural("software"), Some("software"));
assert_eq!(unique_name_of_plural("titles"), None);