18 is congruent to 42 modulo 12 because both 18 and 42 leave 6 as a remainder when divided by 12.
This proposal is the best so far, modulo the fact that parts of it need modification.
21 and 84 are congruent to each other modulo 9, since both numbers leave the same remainder, 3, when divided by 9.
Thus 21 modulo 9 is 3, because when 21 is divided by 9, the remainder is 3.
"A is the same as B modulo C" means A is the same as B except for differences accounted for by C.
The "tail recursion modulo cons" technique allows functions that would be tail recursive but for a cons call to be transformed into a tail recursive form.
All mammals, modulo the monotremes, give birth to live young.