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;
reserve L,L1 for Sequence;

theorem
  X c= Y implies for L being Sequence of X holds L is Sequence of Y
proof
  assume
A1: X c= Y;
  let L be Sequence of X;
  rng L c= X by RELAT_1:def 19;
  then rng L c= Y by A1;
  hence thesis by RELAT_1:def 19;
end;
