An illegal pattern and a computer program having a method are received. The method has one or more return statements, and a number of basic blocks. The method is normalized so that each return statement of the target method relating to the illegal pattern returns a constant Boolean value. A first path condition and a second path condition for one or more corresponding paths is determined such that one or more corresponding basic blocks return a constant Boolean value of true for the first path condition and a constant Boolean value of false for the second path condition. An unsatisfiability of each path condition is determined using a monadic second-order logic (M2L) technique. Where the unsatisfiability of either path condition is false, the method is reported as not being a validator. Where the unsatisfiability of either path condition is true, the method is reported as being a validator.