theorem Th91:
  x1 in P & x2 in P & x3 in P implies plane(x1,x2,x3) c= P
proof
  P in plane_of_REAL n;
  then
A1: ex P9 being Subset of REAL n st P = P9 & ex y1,y2,y3 being Element of
  REAL n st P9 = plane(y1,y2,y3);
  assume x1 in P & x2 in P & x3 in P;
  hence thesis by A1,Th83;
end;
