Modulus 11 can be implemented in different ways. This implementation is made according to the description in this document (english description starts on page 4).
Modulus 11 is a method to calculate a check digit. Check digits helps detect human transcription errors, for example in bank account numbers. More about check digits can be read here.
import Mod11
Mod11.calculateCheckDigit [2, 4, 1, 3, 5] == Ok 0
Mod11.hasValidCheckDigit [2, 4, 1, 3, 5, 0] == Ok True