reserve X,Y,Z,X1,X2,X3,X4,X5,X6 for set, x,y for object;
reserve a,b,c for object, X,Y,Z,x,y,z for set;
reserve A,B,C,D for Ordinal;
reserve F,G for Function;

theorem
  {} is Sequence of Z
proof
  rng {} c= Z;
  hence thesis by RELAT_1:def 19;
end;
