public void PerformHeadMove(Direction d)
{
Debug.Assert(Alive);
Debug.Assert(world.fields[Head.Y, Head.X].occupant == FastWorld.Occupant.Snake);
Debug.Assert(world.fields[Head.Y, Head.X].id == index);
// Write direction we left field in to old head position
world.fields[Head.Y, Head.X] = world.fields[Head.Y, Head.X].WithModifiedDirection(d);
Head = Head.Advanced(d);
// Assert that the tile we are moving into is actually empty at this point
Debug.Assert(world.fields[Head.Y, Head.X].occupant == FastWorld.Occupant.Empty);
world.fields[Head.Y, Head.X] = new FastWorld.Field(FastWorld.Occupant.Snake, index, d);
++Length;
}