reserve A,A1,A2,B,C,D for Ordinal,
  X,Y for set,
  x,y,a,b,c for object,
  L,L1,L2,L3 for Sequence,
  f for Function;
reserve fi,psi for Ordinal-Sequence;

theorem Th25:
  A in dom fi implies fi.A is Ordinal
proof
  assume A in dom fi;
  then
A1: fi.A in rng fi by FUNCT_1:def 3;
  ex B st rng fi c= B by Def4;
  hence thesis by A1;
end;
