Function merge_part_as_ossia 

pub fn merge_part_as_ossia(
    main: &Stream,
    ossia: &Stream,
    ossia_name: &str,
    compare_by_measure_number: bool,
    recurse_in_measures: bool,
) -> Result<Stream>

Merges the measures of an ossia part that hold notes into a copy of a part, each as a variant named ossia_name where the ossia’s measure stands – the measure of the same number, with compare_by_measure_number, or else at the same offset – or, with recurse_in_measures, each stretch of its notes that differs as a variant inside the part’s measure: music21’s mergePartAsOssia.

Errors

No measure of the part where the ossia has one, or as merge_variants_equal_duration.