Exercise - Roman Numerals

Roman numerals can be used to represent numbers:

1 = I
4 = IV
5 = V
9 = IX
10 = X
50 = L
100 = C
500 = D
1000 = M

Write a function that can convert a number (up to 3000) into Roman numerals.

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Try doing this as a TDD exercise. What are the key test cases?