To explain my idea further :
1. Pawns are not promoted at all by the Nietzsche rule. However, when they reach the 8th rank, they are promoted as usual, and only then enter the promotion sequence.
2. After (1.e4 e5 2.Nf3 Nf6), the e5 pawn is attacked once, and not defended, so it is promoted to a Knight (assuming suggestion 1 is not in effect. Consider this : (1.e4 e5 2.Nf3 Nc6) the pawn doesn't promote, because it is now attacked once and defended once. If the game was to continue (3.d4 Nf6) the pawn at e5 promotes, because it's attacked twice and defended once.
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The only objection I have about this mutator is its clarity. It appears to be difficult to see more than one move ahead. If both suggestions above are taken into account, it will make things a little easier.