reserve x for object,
  a,b for Real,
  k,k1,i1,j1,w for Nat,
  m,m1,n,n1 for Integer;
reserve p,q for Rational;

theorem
  -numerator(p) = denominator(p) iff p = -1
proof
  -numerator(p)=denominator(p) iff numerator(p)=-denominator(p);
  hence thesis by Th21;
end;
