[ refactor] Using `otherwise` guard instead of pattern matching

Style teacher and me agree on using

  f pattern
    | predicate1 = foo1
    | predicate2 = foo2
    | ...
    | predicateN = fooN
    | otherwise  = bar

instead of

  f pattern
    | predicate1 = foo1
    | predicate2 = foo2
    | ...
    | predicateN = fooN

  f pattern = bar