Oh yeah? Read the proof again. It's okay for the product to be negative zero! I am not saying this is incorrect. I am making no assumptions: I showed that -1 * 0 = 0 and then I showed that -1 * 0 = -0 and by transitivity of equality (what this means is that the left hand side which is the same equals the rhs of 0 and -0 respectively, therefore 0 and -0 must be the same) these are equal. No rocket science required here Melchoir.