reserve a,a1,a2,a3,b,b1,b2,b3,r,s,t,u for Real;
reserve n for Nat;
reserve x0,x,x1,x2,x3,y0,y,y1,y2,y3 for Element of REAL n;
reserve L,L0,L1,L2 for Element of line_of_REAL n;
reserve P,P0,P1,P2 for Element of plane_of_REAL n;

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;
