This question about typesetting pullback square has almost everything I want, except the placement of the 'corner' symbols to be in the corner of the diagram, and not in the center.
My problem a conflict between placing the label in the middle of the empty arrow, i.e using \ar@{}[dr]|{\ulcorner}, and moving the label to the very end of the arrow as on the bottom right corner of page 4 of the xypic manual, achieved by \ar[r]^>{\ulcorner}.
Writing out \ar{}[dr]^>|{\ulcorner} does not give the desired result. How can I move a label along the an empty arrow?