def get_sharemap():
    """Return a dict mapping share identifier to set of IServer
        instances. This indicates which servers were given which shares. For
        immutable files, the shareid is an integer (the share number, from 0
        to N-1). For mutable files, it is a string of the form
        'seq%d-%s-sh%d', containing the sequence number, the roothash, and
        the share number."""
