Confusingly, the explanations uses two adjectives: "equivalent" and "identical".
Text should say: _"A variable is only equivalent
to a sharing variable"_.
Examples:
X=f(a),Y=f(a), X==Y : true X=f(a),Y=X, X==Y : true X==Y : false X=Y, X==Y : true X=A, Y=A, X==Y : true A=B, X=A, Y=B, X==Y : true A=f(X), B=f(X), A==B : true A=f(X), B=f(Y), A==B : false A=f(X), B=f(Y), X=Y, A==B : true A=f(X), B=A, A==B : true