<dd>either an <i>r</i> or an <i>s</i></dd>
<dt><i>r</i>/<i>s</i></dt>
<dd>an <i>r</i> but only if it is followed by an <i>s</i>. The <i>s</i> is not part of
-the matched text. This type of regular expression is called "trailing
-context".</dd>
+the matched text. This type of regular expression is called "trailing context". A trailing
+context can only be the end of a rule and not part of a named definition.</dd>
<dt><i>r</i>{<i>n</i>}</dt>
<dd>matches <i>r</i> exactly <i>n</i> times.</dd>
<dt><i>r</i>{<i>n</i>,}</dt>
.TP
\fIr\fP\fC/\fP\fIs\fP
an \fIr\fP but only if it is followed by an \fIs\fP. The \fIs\fP is not part of
-the matched text. This type of \*(rx is called "trailing context".
+the matched text. This type of \*(rx is called "trailing context". A trailing
+context can only be the end of a rule and not part of a named definition.
.TP
\fIr\fP\fC{\fP\fIn\fP\fC}\fP
matches \fIr\fP exactly \fIn\fP times.