Function interp_btreemap_keys 

fn interp_btreemap_keys(
    args: &[RValue],
    named: &[(String, RValue)],
    context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Return all keys of a BTreeMap as a sorted character vector.

@param h integer scalar: btreemap ID @return character vector of keys in sorted order