[munchler]

If sqrt -1 = 0, then (by squaring both sides) -1 = 0, which is clearly unsound.

[sebzim4500]

Right but there isn't a theorem saying `(sqrt x)^2 = x`, there's a theorem saying `x >= 0 -> (sqrt x)^2 = x`

[munchler]

Ah, that makes sense. Thank you. As long as every use of sqrt has such a condition.