Synchronized extension systems (SE-systems, for short) are 4-tuples G = (V,
L-1, L-2, S), where V is an alphabet and L-1, L-2 and S are languages over
V. They generate languages extending L-1 by L-2 to the left or to the righ
t, and synchronizing on words in S. Such systems appear naturally when cons
idering stacks, queues, grammar-like generative devices, splicing systems,
zigzag-codes etc.