Function compose 

pub fn compose(
    paths: &[impl AsRef<Path>],
    options: Options,
    keep_going: &mut impl FnMut(usize, &Path) -> bool,
) -> Result<Option<(RgbaImage, bool)>, String>

Draws the pictures at paths on one canvas. keep_going hears of each one before it is read, and returning false stops with Ok(None).