Tests do specific instances of a class of problem and proves that your implementation is correct for these instances. Formal verification proves the whole class

Tests do specific instances of a class of problem and proves that your implementation is correct for these instances. Formal verification proves the whole class. You can have a function that returns the anagram and testing will proves it correct for some pairs of words. But to prove it for all words require formal verification. And that’s when you catch some tricky errors due to undefined behavior or library bugs because you can’t prove their semantics.