reserve a,b,c,d,x,y,z for object, X,Y,Z for set;
reserve R,S,T for Relation;
reserve F,G for Function;

theorem Th9:
  R-Seg(a) c= field R
proof
  let b be object;
  assume b in R-Seg(a);
  then [b,a] in R by Th1;
  hence thesis by RELAT_1:15;
end;
