qarrow draws an arrow head pointing to the current anchor set by qat.
qarrow(l, w) specifies length and (full) width of the arrow head
These are specified in points, and default to l=8, w=5.
qarrow(l, w, dl) specifies that the arrow is to be displaced from the anchor by a distance dl along the arrow's axis.
qarrow(l, w, dl, dimple) specifies that the back of the arrow head is indented by dimple points.
qarrow(l, w, dl, dimple, dw) specifies that the arrow is to be displaced from the anchor by dw points in the orthogonal direction of the arrow's axis.