code: func isForkIncompatible(s1, s2, head *big.Int) bool {
  	return (isForked(s1, head) || isForked(s2, head)) && !configNumEqual(s1, s2)
  }
docs: // isForkIncompatible returns true if a fork scheduled at s1 cannot be rescheduled to
  // block s2 because head is already past the fork.