append/3 - concatenates two lists - back to content

		?- append([a,b],[c],R).
		R=[a,b,c]
		?- append(_,[e,s],[f,o,x,e,s]).
		yes
		?- append(X,[e,s],[f,o,x,e,s]).
		X=[f,o,x]
		?-append(X,Y,[a,b,c]).
		X=[]
		Y=[a,b,c]
     		X=[a]
     		Y=[b,c]
     		X=[a,b]
     		Y=[c]
     		X=[a,b,c]
     		Y=[]
     		no

		append([],L,L).
		append([H|T],L,[H|R]):- append(T,L,R).
	
		Example. last3/2 finds the last 3 elements of the list.

     		?-last3([c,o,l,l,e,g,e],R).
		R=[e,g,e]

		last3(L,[A1,A2,A3]):- append(_,[A1,A2,A3],L).
		or
		last3(L,[A1,A2,A3]):- append(_,X,L), length(X,3).