
The arrow head gets broken when touching the target. You have to shift the whole arrow down until it fits again. If you replace the fraction by just A, it works.
You have just the possibility to shift the arrow like in \ar@&lt;-1ex&gt; or to increase the radius. I would not do both but just increase the vertical height of the first cell.
I loaded amsmath and \dfrac works as expected. \xymatrix gets set in inline math style. No matter if you put it in $$ or in \[\] ([preview] option to standalone needed)


Here is my approach. I just found out that the depth of the first cell has to be a little bit bigger than the depth of the last cell. The bigger the subscript, the bigger the distance between s and \dfrac{A}{B}

% arara: pdflatex

\documentclass[12pt]{standalone}
\usepackage[all,cmtip]{xypic}
\usepackage{mathtools}

\begin{document}
$\xymatrix{%
        A\vphantom{\dfrac{A}{B_x}} \ar[r]  \ar@&lt;0ex&gt; `d/8pt[r] `[rr]_s [rr]
        &amp;
        \dfrac{A}{B} \ar[r]
        &amp;
        \dfrac{A}{B}
    }$
\end{document}




The result may not be pleasing due to the big gap on the left. Will be better when just using \frac{}{}. And, you guessed it, it would be nicer with tikz-cd!

% arara: pdflatex

\documentclass[12pt, border=2mm]{standalone}
\usepackage{tikz-cd}
\usepackage{mathtools}

\begin{document}
\begin{tikzcd}
    A \arrow{r} \arrow[rounded corners, to path={ -- ([yshift=-3ex]\tikztostart.south)\tikztonodes -| (\tikztotarget.south)}]{rr}{s} &amp; \dfrac{A}{B} \arrow{r} &amp; \dfrac{A}{B}
\end{tikzcd}
\end{document}




Arrow tip for curved arrow in xymatrix
I’m using Lightroom 5.7 on OS X. Last night I imported my iPhoto library using Lightroom’s “File > Plug-in Extras > Import from iPhoto Library…” command. The photos all seem to have been imported, but only the original versions. There’s a collection called “Photos Adjusted in iPhoto”, but I’m not convinced that even the photos in here reflect the adjustments I made in iPhoto.

I would have expected each edited photo to have been imported in its original form and then had an “Edited in iPhoto” entry added in the History to reflect all of the iPhoto changes. Am I missing something, or is this functionality missing in Lightroom’s iPhoto importer? How can I import my library and bring along all of my adjustments too?
