diff -r 494b44b439bf -r 2f9fe225ecc8 hws/hw01.tex
--- a/hws/hw01.tex	Thu Dec 31 23:40:48 2015 +0000
+++ b/hws/hw01.tex	Tue Jan 05 01:46:41 2016 +0000
@@ -38,7 +38,8 @@
 \item Assume the concatenation operation of two strings is
       written as $s_1 @ s_2$. Define the operation of
       \emph{concatenating} two sets of strings. This operation
-      is also written as $\_ \,@\, \_$.
+      is also written as $\_ \,@\, \_$. According to 
+      this definition, what is $A \,@\, \{\}$ equal to?
 
 \item Assume a set $A$ contains 4 strings and a set $B$
       contains 7 strings. None of the strings is the empty