What is the applicable rule for reimbursing a party who uses their separate property for community purposes?

Both parties argue that the applicable rule can be found in See v. See (1966) 64 Cal.2d 778, 51 Cal.Rptr. 888, 415 P.2d 776. In that case, this court stated that "the party who uses his [or her] separate property for community purposes is entitled to reimbursement from the community or separate property of the other only if there is an agreement between the parties to that effect." (Id., at p. 785, 51 Cal.Rptr. 888, 415 P.2d 776.)