document st@rred flag for xxalignat, fixes #159

required/amsmath/amsmath.dtx

@@ -3197,9 +3197,10 @@ ill-advised in LaTeX.%
 % \end{macro}
 %
 % \begin{macro}{\ifst@rred}
-%    The switch |st@rred| is set to true by all starred
-%    environments
-%    and set to false by the unstarred versions.
+%   The switch |st@rred| is set to true by all starred environments
+%   and set to false by the unstarred versions.
+%   One exception is the \env{xxalignat} environment where this is set
+%   to true.
 %    \begin{macrocode}
 \newif\ifst@rred
 %    \end{macrocode}
@@ -3450,7 +3451,8 @@ and fix things up.}
             \nointerlineskip
             \vskip-\ht\strutbox@
             \vskip-\dp\strutbox@
-            \vbox{\noindent\hbox to\displaywidth{\hbox to#1{\strut@\hfill}}}%
+            \vbox{\noindent\hbox to\displaywidth{%
+                           \hbox to#1{\strut@\hfill}}}%
             \prevdepth\dimen@
         \fi
     }%