theorem Th20:
  for P being Subset of TOP-REAL 2, p1,p2,q1 being Point of TOP-REAL 2
  holds R_Segment(P,p1,p2,q1) c= P
proof
  let P be Subset of TOP-REAL 2, p1,p2,q1 be Point of TOP-REAL 2;
  let x be object;
  assume x in R_Segment(P,p1,p2,q1);
  then ex q st q=x & LE q1,q,P,p1,p2;
  hence thesis;
end;
