theorem Th5:
  x,y // z,t & x,y // t,z implies x=y or z=t
proof
  assume that
A1: x,y // z,t & x,y // t,z & x<>y and
A2: z<>t;
  z,t // t,z by A1,ANALOAF:def 5;
  hence contradiction by A2,ANALOAF:def 5;
end;
