Hi Xuba.

Xuba wrote:it is not a Forcing Chain; it is Colouring

Colouring is in fact a special type of forcing chain.

CathyW wrote:Colouring conjugate 1s also solves the puzzle.

Here is another solution, similar to that colouring of "1s":

Let's consider that r7c4 is not "1": in that case, we would have a Deadly Turbot Fish in cells r6c4/r1c4/r1c1/r4c1/r6c3, where according to the TIL Argument we could write simultaneously 1) [r6c4]-1-[r6c3]=1=[r4c1]-1-[r1c1]=1=[r1c4]-1-[r6c4], => r6c4<>1, and 2) [r6c4]=1=[r6c3]-1-[r4c1]=1=[r1c1]-1-[r1c4]=1=[r6c4], => r6c4=1. This is a contradiction situation, and so we must have r7c4=1 which solve the puzzle.

Carcul