We study the class of operations definable from the given operations of an algebra of sets by union, composition, and fixed points; we obtain two theorems on definable operations that give us as special case the regular-equals-recognisable theorem of generalised finite automata theory. Definable operations arise also as the operations computable by charts; by translating into predicate logic, we obtain Manna’s formulas for termination and correctness of flowcharts.