clarify refinestep

language2.tex

@@ -754,7 +754,8 @@ From a formal point of view, this means to give up the completeness of the algor
         %\tyof {e'} \Gamma \tsand
         \bigwedge_{\{\varpi \alt \occ e \varpi \equiv e'\}}
         \env {\Gamma,e,t} (\varpi) & \text{if } \exists \varpi.\ \occ e \varpi \equiv e' \\
-        \Gamma(e') & \text{otherwise}
+        \Gamma(e') & \text{otherwise, if } e'\in\dom\Gamma\\
+        \text{undefined} & \text{otherwise}
       \end{array}\right.
 \end{array}
 \]