if a == nil || b == nil {
		return a == b
	}
	// a != nil && b != nil
	return a.path == b.path
}

// An ifacePair is a node in a stack of interface type pairs compared for identity.
type ifacePair struct {
	x, y *Interface
	prev *ifacePair
}

func (p *ifacePair) identical(q *ifacePair) bool {
	return p.x == q.x && p.y == q.y || p.x == q.y && p.y == q.x
}

// A comparer is used to compare types.

	if a == nil || b == nil {
		return a == b
	}
	// a != nil && b != nil
	return a.path == b.path
}

// An ifacePair is a node in a stack of interface type pairs compared for identity.
type ifacePair struct {
	x, y *Interface
	prev *ifacePair
}

func (p *ifacePair) identical(q *ifacePair) bool {
	return p.x == q.x && p.y == q.y || p.x == q.y && p.y == q.x
}

// A comparer is used to compare types.

// adapted version of Checker.identical. For changes to that
// code the corresponding changes should be made here.
// Must not be called directly from outside the unifier.
func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
	u.depth++
	if traceInference {
		u.tracef("%s ≡ %s\t// %s", x, y, mode)
	}
	defer func() {
		if traceInference && !result {
			u.tracef("%s ≢ %s", x, y)
		}
		u.depth--
	}()

// adapted version of Checker.identical. For changes to that
// code the corresponding changes should be made here.
// Must not be called directly from outside the unifier.
func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
	u.depth++
	if traceInference {
		u.tracef("%s ≡ %s\t// %s", x, y, mode)
	}
	defer func() {
		if traceInference && !result {
			u.tracef("%s ≢ %s", x, y)
		}
		u.depth--
	}()