Hole type: subroutine
Adding up the digits of a non-negative integer produces a number:
1234567 -> 1+2+3+4+5+6+7 -> 28
Adding up those digits produces another:
28 -> 2+8 -> 10
Repeating this process until only a single digit is left produces a checksum known as a digital root:
10 -> 1+0 -> 1
Write a subroutine to compute the digital root of any non-negative integer argument.