reserve W for Universe,
  H for ZF-formula,
  x,y,z,X for set,
  k for Variable,
  f for Function of VAR,W,
  u,v for Element of W;
reserve F for Function,
  A,B,C for Ordinal,
  a,b,b1,b2,c for Ordinal of W,
  fi for Ordinal-Sequence,
  phi for Ordinal-Sequence of W,
  H for ZF-formula;
reserve psi for Ordinal-Sequence;
reserve L for DOMAIN-Sequence of W,
  n for Element of NAT,
  f for Function of VAR,L.a;

theorem Th15:
  a in dom L
proof
  a in On W by ORDINAL1:def 9;
  hence thesis by Def2;
end;
