t1 = g(_, _)
...
tj = g(_, _)

o1 = _
...
ok = _

t1 = g(i1, i2)       t1 = g(i3, i4)
t2 = g(i3, i4)       t2 = g(i1, i2)
t3 = g(t1, t2)       t3 = g(t2, t1)
o = t3               o = t3

t1 = g(i1, i2)       t1 = g(i1, i3)
o = i1               o = i1