Put an element in front of each element of a given list.

		?- inser(a,1,[2,1,3,4],R).
		R = [2,a,1,3,4]
		?- append(X,Y,[a,b]).
		X = []
		Y = [a,b]
		X = [a,b]
		Y = []
  	        X = [a]
		Y = [b]

	Solution 1:
		inser(X,A,[],[]).
		inser(X,A,[A|T],[X,A|T]):-!.
		inser(X,A,[H|T],[H|R]):-inser(X,A,T,R). 

	Solution 2:
		inser(X,A,L,R):- append(P1,[A|P2],L),append(P1,[X,A|P2],R).

		append([],L,L).
		append([H|T],L,[H|R]):- append(T,L,R).