RESTRICTED USE OF THE SPLICING OPERATION

Splicing is a new powerful tool, stemming originally from molecular ge netics but investigated extensively also in language theory. In this p aper we investigate variants of splicing inspired partly by regulating mechanisms customarily studied in language theory, partly by imposing restrictions on the pairs to be spliced or on the result of splicing. The Chomsky hierarchy constitutes a very suitable test bed for the re sulting families, because it is classical and well understood. In cont rast to the usual, nonrestricted splicing, we find several cases when the families of regular or of context-free languages are not closed un der the new types of splicing. On the other hand, our results give new characterizations for families in the Chomsky hierarchy and for closu re properties in general.