In this Part "couple" means—
two people who are married to, or civil partners of, each other and are members of the same household; or
two people who are not married to, or civil partners of, each other but are living together as if they were a married couple or civil partners.
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For the purposes of this section regulations may prescribe—
circumstances in which the fact that two persons are married or are civil partners is to be disregarded;
circumstances in which two people are to be treated as living together as if they were a married couple or civil partners;
circumstances in which people are to be treated as being or not being members of the same household.