The following is a hypothetical situation. I am curious as to what arguments each boat can and should make at the end of the day:
Conditions: Light wind. Constant wind speed and direction. No current. No waves.
Incident: Two boats (A and B) are approaching a windward mark to be rounded to port. Boat A is on starboard and is on her lay line to make the mark without altering course. B is on port and decides to tack underneath A to round the mark on the inside of A. Once B is on her close hauled course it is apparent she must sail above close hauled to round the mark and she does so. At some point while rounding the mark, B sails into the mark. A protests B for touching the mark and breaking rule 31. B does not counter with a protest of any sort on throughout the incident.
After racing: A and B both agree that it debatable whether B tacked inside A before they entered the zone or after. Both A and B agree that B touched the mark. B then brings forward the argument that A did not give B room to round the mark according to rule 18.