sum (consecutive_differences(2, 4, 5, 7))
        = {definition of consecutive differences}
sum((2 - 4), (4 - 5), (5 - 7))
        = {definition of sum}
((2 - 4) + (4 - 5) + (5 - 7))
        = {subtraction equals addition of negative}
(2 + -4 + 4 + -5 + 5 + -7)
        = {associativity and symmetry of addition}
(2 + (4 - 4) + (5 - 5) - 7)
        = {removal of zero terms}
(2 - 7)
        = {arithmetic}
-5