Function: sumdigits
Section: number_theoretical
C-Name: sumdigits0
Prototype: GDG
Help: sumdigits(n,{B=10}): sum of digits in the integer n, when written in
 base B.
Doc: sum of digits in the integer $n$, when written in base $B$.
 \bprog
 ? sumdigits(123456789)
 %1 = 45
 ? sumdigits(123456789, 2)
 %2 = 16
 ? sumdigits(123456789, -2)
 %3 = 15
 @eprog\noindent Note that the sum of bits in $n$ is also returned by
 \tet{hammingweight}. This function is much faster than
 \kbd{vecsum(digits(n,B))} when $B$ is $10$ or a power of $2$, and only
 slightly faster in other cases.
Variant: Also available is \fun{GEN}{sumdigits}{GEN n}, for $B = 10$.
