pub fn merge_map<'buf, K, V, M: Map<K, V>>(
    this: &mut M,
    stream: &mut InputStream<'buf>,
    kmapper: fn(_: &mut InputStream<'buf>, _: &mut K) -> Result<()>,
    vmapper: fn(_: &mut InputStream<'buf>, _: &mut V) -> Result<()>
) -> Result<()>
where K: Default, V: Default,