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Training puzzle does not accept equivalent mates

Hi,

I just played this puzzle : http://en.lichess.org/training/40538. It refused as incorrect the variant : 1...Rg2+ 2.Kh3 Rg4 3.Kh2 Kf2 4.Kh3 Rh4#. Instead it showed a solution that is another mate in 4. I checked my variant with stockfish and it is indeed one of the three M4 possible in the position, here is its output.

[M4] 1... Ne3 2. Rxd6 Ng4+ 3. Kh1 Nf2+ 4. Kh2 Rg2# (depth 87, 0:00:18)
[M4] 1... Rg2+ 2. Kh3 Rg4 3. Kh2 Kf2 4. Ra7 Rh4# (depth 87, 0:00:18)
[M4] 1... Nh6 2. Rxd6 Ng4+ 3. Kh1 Nf2+ 4. Kh2 Rg2# (depth 87, 0:00:18)
[M5] 1... Kf2 2. Rxd6 Nxd6 3. b4 Rg4 4. Kh3 Nf5 5. bxc5 Rh4# (depth 86, 0:00:18)

Where the first variant is the accepted solution, the 2 others are deemed incorrect and the 4th is treated as "good move but there is better".

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