theorem Th26:
  ex t st x,y '||' z,t & x,z '||' y,t
proof
  consider t such that
A1: x,y // z,t & x,z // y,t and
  y<>t by ANALOAF:def 5;
  x,y '||' z,t & x,z '||' y,t by A1;
  hence thesis;
end;
