Our definition "digit"

a symbol denoting/representing a natural number ("a numeral")
we'll work with four systems of numerals
1. roman: I, V, X, L, C, D, M
2. decimal: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
3. binary: 0, 1
4. hexadecimal: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f
other systems of numerals are also possible (e.g. base64)

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