Married parents. Parent A's a WOHP Mon-Fri, parent B's a SAHP. Childcare shared 50/50 when A's at home. Parent A had a work "crisis" requiring them to work a weekend, so parent B had to do childcare all weekend. Parent A says that this was a "joint sacrifice". Parent B feels that they did parent A a favour, and A should (e.g.) do all the childcare next weekend. Who is BU?