HoDoKu {0}
{3}
Coloring is a unary (single value) forcing chain of regions with two places for
{1}, so {1} must either be in the greens or the blues.
Hence any {1} that sees two colored {1}'s is history:
- color trap: any candidate which can see two cells of opposite colors
can be removed.
- color wrap (rarer): if two cells of the same color see each other,
then all candidates of that color can be removed.
Therefore we can remove {2}.