下列内容为何编译失败？
\documentclass[twocolumn, 10pt]{article}
\usepackage{mathptmx}
\usepackage[subtle]{savetrees}
\title{Test}
\author{Foo}
\date{}
\begin{document}
\maketitle
\section{Introduction}
Foo foo foo
\end{document}
如果或被mathptmx注释掉savetrees，pdflatex 将成功编译该文件。
这是编译日志：
This is pdfTeX, Version 3.14159265-2.6-1.40.17 (TeX Live 2016) (preloaded format=pdflatex)
restricted \write18 enabled.
entering extended mode
(./main.tex
LaTeX2e <2016/03/31>
Babel <3.9r> and hyphenation patterns for 83 language(s) loaded.
(/usr/local/texlive/2016/texmf-dist/tex/latex/base/article.cls
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
(/usr/local/texlive/2016/texmf-dist/tex/latex/base/size10.clo))
(/usr/local/texlive/2016/texmf-dist/tex/latex/psnfss/mathptmx.sty)
(/usr/local/texlive/2016/texmf-dist/tex/latex/savetrees/savetrees.sty
(/usr/local/texlive/2016/texmf-dist/tex/latex/xkeyval/xkeyval.sty
(/usr/local/texlive/2016/texmf-dist/tex/generic/xkeyval/xkeyval.tex
(/usr/local/texlive/2016/texmf-dist/tex/generic/xkeyval/xkvutils.tex
(/usr/local/texlive/2016/texmf-dist/tex/generic/xkeyval/keyval.tex))))
(/usr/local/texlive/2016/texmf-dist/tex/generic/oberdiek/ifpdf.sty)
(/usr/local/texlive/2016/texmf-dist/tex/generic/oberdiek/ifluatex.sty))
(/usr/local/texlive/2016/texmf-dist/tex/latex/microtype/microtype.sty
(/usr/local/texlive/2016/texmf-dist/tex/latex/microtype/microtype-pdftex.def)
(/usr/local/texlive/2016/texmf-dist/tex/latex/microtype/microtype.cfg))
(./main.aux) (/usr/local/texlive/2016/texmf-dist/tex/latex/psnfss/ot1ptm.fd)
(/usr/local/texlive/2016/texmf-dist/tex/latex/microtype/mt-ptm.cfg)
(/usr/local/texlive/2016/texmf-dist/tex/latex/psnfss/ot1ztmcm.fd)
(/usr/local/texlive/2016/texmf-dist/tex/latex/psnfss/omlztmcm.fd)
(/usr/local/texlive/2016/texmf-dist/tex/latex/psnfss/omsztmcm.fd)
(/usr/local/texlive/2016/texmf-dist/tex/latex/psnfss/omxztmcm.fd)
! TeX capacity exceeded, sorry [grouping levels=255].
pickup@font ...relax define@newfont fi begingroup
escapechar m@ne MT@let@cn ...
l.11 \maketitle
! ==> Fatal error occurred, no output PDF file produced!
Transcript written on main.log.
谢谢！