Function stage 

pub fn stage(
    paths: &[impl AsRef<Path>],
    options: Options,
    import: &ImportOptions,
    keep_going: impl FnMut(usize, &Path) -> bool,
) -> Result<Option<Combined>, String>

Combines the pictures and stages the PNG for import with import. See compose for keep_going; it also hears of the save, as step paths.len().