Definition
17
[1X2] Given two variables \(x,y\) we will write \(xβ y\) to say that β\(x\) is an element of the set \(y\)β. Equivalent expressions are β\(x\) is a member of \(y\)β, β\(x\) belongs to \(y\)β or just simply β\(x\) is in \(y\)β.
The formula \((xβ y)\) is equivalent to \((yβ x)\); the negations are \((xβ y )β Β¬ (xβ y) \) and \((y β x)β Β¬ (yβ x)\).
The formula \((xβ y)\) (as all other variants) takes value of truth/falsehood and therefore can be used as atom in the construction of a well-formed formula.