  In the paper the problem of verification of functional programs (FPs) over
strings is considered, where specifications of properties of FPs are defined by
other FPs, and a FP S1 meets a specification defined by another FP S2 iff a
composition of functions defined by the FPs S1 and S2 is equal to the constant
1. We introduce a concept of a state diagram of a FP, and reduce the
verification problem to the problem of an analysis of the state diagrams of
FPs. The proposed approach is illustrated by the example of verification of a
sorting program.
