typos

related.tex

@@ -460,7 +460,7 @@ expressions with different marks are to be considered as different
 expressions and, therefore, would not share the same type
 hypothesis. For instance, consider the test
 $\ifty{f\,x}\Int{\,...\,}{\,...}$: if $f x$ were flagged as impure
-then the occurrence of $f x$ in the ``then'' branch would not be
+then an occurrence of $f x$ in the ``then'' branch would not be
 supposed to be of type $\Int$ since it would be typed in an
 environment $\Gamma$ containing a binding for an $f\,x$ expression
 having a mark different from the one in the ``then'' branch: the