The Join(L, sep) function inserts the separator sep between the elements of the list L.

$\mathrm{with}\left(\mathrm{ListTools}\right)\:$

$L≔\left[1\,2\,3\,4\,5\,6\,7\,8\,9\,10\right]$

$L≔\left[1\,2\,3\,4\,5\,6\,7\,8\,9\,10\right]$

$\mathrm{Join}\left(L\,x\right)$

$\left[1\,x\,2\,x\,3\,x\,4\,x\,5\,x\,6\,x\,7\,x\,8\,x\,9\,x\,10\right]$