Function: sumdigits
Section: number_theoretical
C-Name: sumdigits
Prototype: G
Help: sumdigits(n): sum of (decimal) digits in the integer n.
Doc: sum of (decimal) digits in the integer $n$.
 \bprog
 ? sumdigits(123456789)
 %1 = 45
 @eprog\noindent Other bases that 10 are not supported. Note that the sum of
 bits in $n$ is returned by \tet{hammingweight}.
