A General, Verifiable Iterative Control Structure

A general iterative control structure with multiple exits and multiple retums to the beginning of the loop is described. The conditions causing these exits or returns must be declared and defined in a prologue to the loop in a manner which is similar to the declaration of variables at the beginning of a block. Also, actions prior to an exit or a return may be specified in an epilogue of the loop. The semantics of the structure are described informally and then formally by means of a Hoare type verification rule. Attributes of the structure and examples of its usage are given in the latter part of the paper.