Function: vecsum
Section: linear_algebra
C-Name: vecsum
Prototype: G
Help: vecsum(v): return the sum of the components of the vector v.
Doc: return the sum of the components of the vector $v$. Return $0$ on an
 empty vector.
 \bprog
 ? vecsum([1,2,3])
 %1 = 6
 ? vecsum([])
 %2 = 0
 @eprog
