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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_member_ids():
 """Return all member ids of the portal. """

    pm = get_tool("portal_membership")
    member_ids = pm.listMemberIds()
    # How can it be possible to get member ids with None?
    return filter(lambda x: x, member_ids)