theorem
  Line(x1,x2) c= plane(x1,x2,x3) & Line(x2,x3) c= plane(x1,x2,x3) & Line
  (x3,x1) c= plane(x1,x2,x3)
proof
A1: x3 in plane(x1,x2,x3) by Th82;
  x1 in plane(x1,x2,x3) & x2 in plane(x1,x2,x3) by Th82;
  hence thesis by A1,Th85;
end;
