theorem Th99:
  ex P st x in P & L c= P
proof
  L in line_of_REAL n;
  then consider x1,x2 being Element of REAL n such that
A1: L = Line(x1,x2);
  take P = plane(x,x1,x2);
  x1 in P & x2 in P by Th82;
  hence thesis by A1,Th82,Th85,Th90;
end;
