2.7 Arrows passing under
Arrows can pass under (or via) any other entry: Just insert't, i.e., a quote (apostrophe) character
followed by a target, for each entry that should be visited except
the last, "ordinary & final" entry:
\xymatrix{
{\circ}
\ar@{|-->} '[dr] ^a
'[rr]+D*{\bullet} ^b
[drrr] ^c
& {\circ} & {\circ} & {\circ} \\
{\circ} & {\circ} & {\circ} & {\circ} }
typesets

As you see, labels are set separately on each segment.
Exercise 12.
Typeset the "lambda cube"
Hint: "going under" an empty entry leaves a small gap at that location. The compactness is achieved using a trick described in section 3.3.
© Kristoffer H. Rose <krisrose@brics.dk> January 6, 1997 / LaTeX2HTMLv96.1-h