This property of a reduction system states that if an

expression can be reduced by zero or more reduction steps to

either expression M or expression N then there exists some

other expression to which both M and N can be reduced. This

implies that there is a unique normal form for any

expression since M and N cannot be different normal forms

because the theorem says they can be reduced to some other

expression and normal forms are irreducible by definition. It

does not imply that a normal form is reachable, only that if

reduction terminates it will reach a unique normal form.