intSubtract method Null safety

Implementation

void intSubtract(Model m) {
  final Value x = m.x;
  final BigInt xi = x.internal;
  final BigInt yi = m.y.internal;
  _intSubtract(xi, yi, m);
  m.lastX = x;
  m.cFlag = xi > yi;
  // The 16C sets carry whenever a binary subtraction results in a
  // borrow into the most significant bit, and clears it otherwise.
  // That's not the same as what you get by adding the negation.
}