We show that the equality set Eq(g, h) of two non-periodic binary morphisms g, h: A* --> Sigma* is generated by at most two words. If the rank of Eq(g, h) = {alpha, beta}* is two, then alpha and beta begin and end with different letters. This in particular implies that any binary language has a test set of cardinality at most two. (C) 2002 Elsevier Science (USA). All rights reserved. [References: 10]
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