answer(split_missing_separator, ["abc"]).
answer(split_empty_separator, ["a", "b", "c"]).
answer(join_empty, "").
answer(join_numbers, "1-2-3").
answer(substring_empty, "").
answer(replace_empty_search, "abc").
answer(lower_string, "hello").
answer(upper_string, "EYEPL 123").