Plus<T> is a function object. Specifically, it is an
Adaptable Binary Function. If f is an object of class
plus<T> and x and y are objects of class T, then
f(x,y) returns x+y.
Example
Each element in V3 will be the sum of the corresponding elements
in V1 and V2
T must be a numeric type; if x and y are objects of type T,
then x+y must be defined and must have a return type that is
convertible to T. T must be Assignable.