Finish to refactor tests for `two_in_a_row' function.
Move function `sum_of_prefixes' (unprotected) in its own file.
Move function `scramble' in its dedicated file.
Refactor function `multirember' in order to use functors to show
refinements reading chapter two of Seasoned Schemer.
Refactor `multirember' in order to follow Twelfth Commandment.
Introduce `member' function toward `union' function.