reserve x,y,c for set;

theorem
  for x,y,c being non pair set holds InputVertices
  BitSubtracterWithBorrowStr(x,y,c) is without_pairs
proof
  let x,y,c be non pair set;
  InputVertices BitSubtracterWithBorrowStr(x,y,c) = {x,y,c} by Th19;
  hence thesis;
end;
