The Canon Law: Letter and Spirit, a commentary on canon law, explains that condition may be defined as "a stipulation by which an agreement is made contingent upon the verification or fulfillment of some circumstance or event that is not yet certain". It goes on to state that "any condition concerning the future attached to matrimonial consent renders marriage invalid". For example, a marriage would be invalid if the parties stipulated that they must have children or they have the right to divorce and remarry someone else.[citation needed]