Dominosa

The dominoes are there, the borders are gone. Pair the numbers so every domino appears once.

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How to play

The grid holds every domino of a set (0-0 up to n-n) exactly once, numbers showing, borders hidden.
Tap the gap between two neighbouring numbers to join them into a domino; tap again for a cross, again to clear.
No two dominoes may show the same pair of numbers, and every square belongs to exactly one domino.
Sizes: 0–3 (10 dominoes), 0–5 (21) and 0–6 (28, the full set).
Every puzzle has exactly one answer; a hint fixes a wrong join or adds a correct one.