convert 5 to V
convert 6 to VI
convert 2,3,7,8
refactor extract constant I
convert 4
refactor extract constant V
convert 9
refactor extract constant ROMAN_DIGIT_ZERO
convert 0
convert 0
refactor extract method
convert 10,...,90 (multiples of 10)
refactor extract method zehnerstelle
refactor extract simplify condition
convert <1000
refactor zehnerstelle to integer_division
fix bug: L and D were switched