theorem Th23:
  1 in C implies exp(C,A) in exp(C,succ A)
proof
A1: 1*^exp(C,A) = exp(C,A) by ORDINAL2:39;
  assume 1 in C;
  then exp(C,A) in C*^exp(C,A) by A1,Th22,ORDINAL2:40;
  hence thesis by ORDINAL2:44;
end;
