diff -r 21f41e08457d -r ae4708c851ee cws/cw03.tex
--- a/cws/cw03.tex	Wed Dec 21 01:19:25 2016 +0000
+++ b/cws/cw03.tex	Wed Dec 21 03:06:18 2016 +0000
@@ -327,18 +327,17 @@
 \end{itemize}\bigskip  
 
 
+\subsection*{Background}
 
-\noindent
-\textbf{Background} Although easily implementable in Scala, the idea
-behind the derivative function might not so easy to be seen. To
-understand its purpose better, assume a regular expression $r$ can
-match strings of the form $c::cs$ (that means strings which start with
-a character $c$ and have some rest, or tail, $cs$). If you now take the
-derivative of $r$ with respect to the character $c$, then you obtain a
-regular expressions that can match all the strings $cs$.  In other
-words, the regular expression $\textit{der}\;c\;r$ can match the same
-strings $c::cs$ that can be matched by $r$, except that the $c$ is
-chopped off.
+Although easily implementable in Scala, the idea behind the derivative
+function might not so easy to be seen. To understand its purpose
+better, assume a regular expression $r$ can match strings of the form
+$c::cs$ (that means strings which start with a character $c$ and have
+some rest, or tail, $cs$). If you now take the derivative of $r$ with
+respect to the character $c$, then you obtain a regular expressions
+that can match all the strings $cs$.  In other words, the regular
+expression $\textit{der}\;c\;r$ can match the same strings $c::cs$
+that can be matched by $r$, except that the $c$ is chopped off.
 
 Assume now $r$ can match the string $abc$. If you take the derivative
 according to $a$ then you obtain a regular expression that can match