
theorem
  for n be Element of NAT for x,y being FinSequence holds
  InnerVertices ((n+1)-BitSubtracterStr(x,y)) =
  InnerVertices (n-BitSubtracterStr(x,y)) \/
  InnerVertices BitSubtracterWithBorrowStr(x .(n+1), y.(n+1),
  n-BitBorrowOutput(x,y))
proof
  let n be Element of NAT;
  let x,y be FinSequence;
  InnerVertices (n-BitSubtracterStr(x,y) +*
  BitSubtracterWithBorrowStr(x .(n+1), y.(n+1), n-BitBorrowOutput(x, y))) =
  InnerVertices (n-BitSubtracterStr(x,y)) \/
  InnerVertices (BitSubtracterWithBorrowStr(x .(n+1), y.(n+1),
  n-BitBorrowOutput(x, y))) by FACIRC_1:27;
  hence thesis by Th7;
end;
