Choices of Convenient Sets

Proceeding in the theory with extensionality, comprehension for classes, existence of the empty set and the assumption the addition of one element to a set makes again a set (others usual set-theoretical operations are not required) we show a week assumption which guarantees existence of a saturated elementary extension (with absolute ?) of the system of hereditarily finite sets. Mathematics Subject Classification: 03C62, 03C50, 03E70.