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