def migration_matrix_single_extant_deme(ndemes, focal_deme):
    """
    Sets up a migration matrix for a system of ndemes
    but where only one deme has a size > 0.  This
    is a convenience function for simulations where
    demes will appear later on and have migration amongst them.

    :param ndemes: The number of demes that will eventually exist.
    :param focal_deme: The index of the initial extant deme.
    :rtype: numpy.ndarray

    .. versionadded:: 0.6.0
    """
    import numpy as np
    m = np.zeros(ndemes * ndemes).reshape(ndemes, ndemes)
    m[focal_deme, focal_deme] = 1.0
    return m
