Function rapply_collect 

fn rapply_collect(
    interp: &Interpreter,
    x: &RValue,
    f: &RValue,
    env: &Environment,
    out: &mut Vec<RValue>,
    classes: Option<&[String]>,
    deflt: Option<&RValue>,
) -> Result<(), RError>

Helper: collect results of applying f to matching leaf (non-list) elements.