`Sum` returns as value the sum of the integers that form (the integer
prefix of) the path.

Sum:<> = 0. Sum:<foo> = 0 foo. Sum:<0> = 0. Sum:<1> = 1. Sum:<1 2 3 4> = 10. Sum:<1 -2 3 -4> = -2. Sum:<1 2 3 4 foo> = 10 foo.

Think of the empty path as a weightless box. Its weight is 0. Put the weightless object (i.e. 0) in it and the weight of both box and contents is 0.