Dear
René:
Indeed the xy.sty is in /usr/local/texlive/2015/texmf-dist/tex/generic/xypic/ as you said.
The project is the writing of a big book on quantum mechanics. It is not easy to sent to you because of other considerations at this time. The problem did not happen until I updated to TextMate beta 8.5. I even reinstalled the texlive 2015 to no avail. Then I switched to use Texmaker 4.4.1, or just run TeXing on a terminal, and there is no TeXing problem what so ever. I have that problem only when using TextMate. I stopped to use TextMate now in order to proceed with my work because I have spent enough time to wrestle with this difficulty. That is my present status.
Thanks for your prompt attention and I hope this problem can be resolved eventually.
With best regards,
Luke W. Mo
Virginia Tech