reserve k for Element of NAT;
reserve r,r1 for Real;
reserve i for Integer;
reserve q for Rational;

theorem
  for C being non empty set, f being PartFunc of C,ExtREAL holds
  dom (1/f) = dom f \ f"{0.} &
  for c being Element of C st c in dom (1/f) holds (1/f).c = 1./(f.c)
by Def9;
