The Pad[N](..., L, ...) function  returns a new list that contains the elements of list L surrounded by the specified objects pd[1], pd[2], ....

In the list that is returned by Pad, each element of L is located at position N in the series of specified objects pd[1], pd[2], ....

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

$L≔\left[a\,b\,c\right]$

$L≔\left[a\,b\,c\right]$

$\mathrm{Pad}\left[2\right]\left(\mathrm{LL1}\,L\,\mathrm{RR1}\,\mathrm{RR2}\right)$

$\left[\mathrm{LL1}\,a\,\mathrm{RR1}\,\mathrm{RR2}\,\mathrm{LL1}\,b\,\mathrm{RR1}\,\mathrm{RR2}\,\mathrm{LL1}\,c\,\mathrm{RR1}\,\mathrm{RR2}\right]$