I have two vectors `x1`

and `x2`

and would like to loop over the elements in `vcat(x1,x2)`

. In particular, I want to sum over the two adjacent elements.

How can I do that without explicitly creating `x = vcat(x1,x2)`

and looping over the elements in `x`

?

In particular, is there a simple solution for the case when there is also a `x3`

and more, and when I want to sum over `q`

elements.

/Paul S