The club's sign up form, door wide open. A stranger changes a number in the address and reads a member's record, without breaking anything. Sign up form. built with an agent. Member list. behind the form. A logged out stranger. changes a number. A member's record. read without breaking in. Three checks. before going live. Sign up form → Member list (fills). A logged out stranger → Member list (queries). Member list → A member's record (returns). Three checks → Member list (closes).