def create_subdirectory(name, initial_children={}, overwrite=True, mutable=
    True, mutable_version=None, metadata=None):
    """I create and attach a directory at the given name. The new
        directory can be empty, or it can be populated with children
        according to 'initial_children', which takes a dictionary in the same
        format as set_nodes (i.e. mapping unicode child name to (childnode,
        metadata) tuples). The child name must be a unicode string. I return
        a Deferred that fires (with the new directory node) when the
        operation finishes."""
