The reasoning is:
      a + (b + 2) = a + (b + (1 + 1)) = a + ((b + 1) + 1) =
= (a + (b + 1)) + 1 = ((a + b) + 1) + 1 = (a + b) + (1 + 1)

5 + (6 + 1) = (5 + 6) + 1
5 + (6 + 2) =