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Let `q` subset **N** x **N** such that
`q`(`x`, `y`) <=> `x` mod 5 = y mod 5.
We have

[ 0]_{q}= {0, 5, 10, 15, 20, 25, ...}, [ 1]_{q}= {1, 6, 11, 16, 21, 26, ...}, [ 2]_{q}= {2, 7, 12, 17, 22, 27, ...}, [ 3]_{q}= {3, 8, 13, 18, 23, 28, ...}, [ 4]_{q}= {4, 9, 14, 19, 24, 29, ...}, [ 5]_{q}= {0, 5, 10, 15, 20, 25, ...}, ...

We see that [`0`]_{q} union [`1`]_{q}
union [`2`]_{q} union [`3`]_{q}
union [`4`]_{q} = **N** and that any two of these sets are
disjoint.

Author: Wolfgang Schreiner

Last Modification: January 12, 2000