
Suppose we have a 2 x n checkerboard (two rows and n columns).
Write the recursive method, dominos, that, given n, returns
the number of possible ways in which we can cover the board with
1 x 2 dominos. For example:
            |
n = 1 -> 1  |
            ||   --
n = 2 -> 2  ||   --
            |||   --|   |--
n = 3 -> 3  |||   --|   |--
If n is less than zero, throw an IllegalArgumentException
You're challenge is to write tests for this problem described above.
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