Used by the routine at arch_check_doorway.

For each of records 0 to 3 with bit 3 of +$07 set -- only the player's two records have it -- within 15 of the arch's middle along both x and y and within 4 of its z, set a nudge of one unit towards the centre line. The nudge goes in +$0E (x) or +$0F (y), which the player's movement adds to its velocity (calc_plyr_dXY) when it next walks. This is why the knight slides into line with an arch as he walks through.