Is it permissible to create a prenuptial agreement that includes clauses about things other than money? For example, it is common in the United States for a marrying couple to come to an agreement about custody of children should the couple divorce. Is this permissible? Can it be written into the ketuba? Can a contract to this effect be signed and brought to a beit din? If the answer to that is "no," is it permissible to create such a document and bring it to a secular court?