Function add_variant 

pub fn add_variant(
    stream: &mut Stream,
    start_offset: FloatType,
    contents: Option<&Stream>,
    variant_name: Option<&str>,
    variant_groups: Option<&[String]>,
    replacement_quarter_length: Option<FloatType>,
)

Adds a variant to a stream at start_offset, holding contents: a measure, the measures of a stream, or, for a stream with none, its elements – each put as far after the variant’s start as start_offset, as music21 puts them. The variant is in variant_groups and variant_name, and stands in for replacement_quarter_length, or for as long as it lasts: music21’s addVariant.

use music21_rs::tinynotation::from_tiny_notation;
use music21_rs::variant::add_variant;

let mut line = from_tiny_notation("4/4 c4 d e f g1")?;
let other = from_tiny_notation("4/4 c4 d e g")?;
add_variant(&mut line, 0.0, Some(other.measures()[0]), Some("draft"), None, None);
assert_eq!(line.events().iter().filter(|event| matches!(
    event.element(),
    music21_rs::StreamElement::Variant(_)
)).count(), 1);