Proof by truth table (F is False, T is True):
p q r (p == q) (q == r) ((p == q) == r) (p == (q == r))
F F F T T F F
F F T T F T T
F T F F F T T
F T T F T F F
T F F F T T T
T F T F F F F
T T F T F F F
T T T T T T T
That proves associativity of (==).
Also note that p /= q ≡ not p == q. Proof:
p q (p /= q) (not p) (not p == q)
F F F T F
F T T T T
T F T F T
T T F F F
And by symmetry of (/=), p /= q ≡ p == not q.
Then:
(p /= q) /= r ≡ (not p == q) == not r ≡ not p == (q == not r) ≡ p /= (q /= r).
Hence (/=) is associative.
Q.E.D.