x=2 => x^2=4, x integer, natural, rational positive, etc.
x<2 => x^2<4, x>2 => x^2>4.
x=2 => x root of X^2-4x+4.
function f(x). f(0)=2. f(x)=2 <=> x=0.
Exist, forall.
positivity. ab<>0, a<b, b<0, a>0, etc.
