xypic.lyx: use a default font except a special one is available

- also add our documentation preamble disclaimer

lib/examples/xypic.lyx

@@ -4,6 +4,15 @@
 \begin_header
 \textclass scrartcl
 \begin_preamble
+% DO NOT ALTER THIS PREAMBLE!!!
+%
+% This preamble is designed to ensure that the file prints
+% out as advertised. If you mess with this preamble,
+% parts of this document may not print out as expected.  If you
+% have problems LaTeXing this file, please contact 
+% the documentation team
+% email: lyx-docs@lists.lyx.org
+
 \usepackage[all]{xy}
 
 % define new commands used in sec. 5.1
@@ -24,6 +33,16 @@
 \def\LyX{\texorpdfstring{%
   L\kern-.1667em\lower.25em\hbox{Y}\kern-.125emX\@}
   {LyX}}
+
+% if pdflatex is used
+\usepackage{ifpdf}
+\ifpdf
+
+% set fonts for nicer pdf view
+\IfFileExists{lmodern.sty}
+ {\usepackage{lmodern}}{}
+
+\fi % end if pdflatex is used
 \end_preamble
 \options BCOR7.5mm
 \use_default_options false
@@ -32,7 +51,7 @@
 \language_package default
 \inputencoding auto
 \fontencoding global
-\font_roman ae
+\font_roman default
 \font_sans default
 \font_typewriter default
 \font_default_family default