The pgf package 3.0.0 update lead to an error being given with
this document. Loading the 'etex' package fixes the problem.
For more information, see:
https://sourceforge.net/p/pgf/bugs/296/
\begin_header
\textclass article
\begin_preamble
+\usepackage{etex}
\usepackage[all, knot]{xy}
\xyoption{arc}
\usepackage{qtree}
\end_layout
\begin_layout LyX-Code
-xy, qtree, tipa, pstricks, pst-plot, rotating, tikz, pgf
+xy, qtree, tipa, pstricks, pst-plot, rotating, tikz, pgf, etex
\end_layout
\begin_layout Standard