  This paper deals with the CREP (Configuration REachability Problem) for
non-uniform cellular automata (CAs). The cells of non-uniform CAs, we have
considered here, can use different Wolfram's rules to generate their next
states. We report an algorithm which decides whether or not a configuration of
a given (non-uniform) cellular automaton is reachable from another
configuration. A characterization tool, named Reachability tree, is used to
develop theories and the decision algorithm for the CREP. Though the worst case
complexity of the algorithm is exponential in time and space, but the average
performance is very good.
