The New Sudoku Players' Forum

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OK for 1. and 2. (obvious)
But 3. is not proven.

OK, so: if 1r1c1 hasn't been asserted as a hidden single, then there are one or more additional cells in r1 (for example), with candidates for '1'.
From (2), T1-delete, allows each of those candidates to be eliminted, leaving a hidden single in r1.

[denis_berthier]

.
OK. That's the missing piece.
So, finally, template-depth 1 isn't an exception and T1bis is as strong as T1.
.