ex4

这是用prolog写的几个程序：程序１是一个旅行社关系系统

notes(et1,fon,14)->;
notes(et1,fon,16)->;
notes(et3,com,16)->;

moyenne(x,y,m)->notes(x,y,z)
		effacer
		preparer(x,y)
	   	calculer(x,y,m);


somme(x,y,s,n)->rule(noteutile(x,y,z),nil)
			   suppress(1)
			   somme(x,y,s1,k1)
			   val(add(s1,z),s)
			   val(add(k1,1),k);
somme(x,y,0,0)->;

calculer(x,y,m)->somme(x,y,s,n)
		val(div(s,n),m);


effacer->rule(noteutile(x,y,z),nil)
	suppress(1)
	fail;
effacer->;


preparer(x,y)->rule(notes(x,y,z),nil)
		assert(noteutile(x,y,z),nil)
		fail;
preparer(x,y)->;