insert({l1,…,ln},i,x)={l1,…,li-1,x,li,…,ln}insert([v1,…,vn],i,x)=[v1,…,vi-1,x,vi,…,vn]where 1<=i<=length(l)+1
polygonal(P1,...,Pi-1,A,Pi,...,Pn) where n=length(P), 1<=i<=n+1