In some parliamentary systems, the head of state retains certain powers in relation to bills to be exercised at his or her discretion. They may have authority to veto a bill until the houses of the legislature have reconsidered it, and approved it a second time; reserve a bill to be signed later, or suspend it indefinitely (generally in states with royal prerogative; this power is rarely used); refer a bill to the courts to test its constitutionality; refer a bill to the people in a referendum.