reserve A,B,C for Ordinal;
reserve a,b,c,d for natural Ordinal;
reserve l,m,n for natural Ordinal;
reserve i,j,k for Element of omega;
reserve x,y,z for Element of RAT+;

theorem Th41:
  a <> {} implies a/a = 1
proof
  assume a <> {};
  then RED(a,a) = 1 by Th27;
  hence thesis by Def10;
end;
