diff --git a/src/roff/troff/input.cpp b/src/roff/troff/input.cpp
--- a/src/roff/troff/input.cpp
+++ b/src/roff/troff/input.cpp
@@ -6446,18 +6446,34 @@ inline int psbb_locator::skip_to_trailer
 //
 // Handle the .psbb request.
 //
 void ps_bbox_request()
 {
+  // Parse input line, to extract file name.
+  //
   symbol nm = get_long_name(1);
   if (nm.is_null())
+    //
+    // No file name specified: ignore the entire request.
+    //
     skip_line();
   else {
+    // File name acquired: swallow the rest of the line.
+    //
     while (!tok.newline() && !tok.eof())
       tok.next();
     errno = 0;
+
+    // Update {llx,lly,urx,ury}_reg_contents:
+    // declaring this class instance achieves this, as an
+    // intentional side effect of object construction.
+    //
     psbb_locator do_ps_file(nm.contents());
+
+    // All done for .psbb; move on, to continue
+    // input stream processing.
+    //
     tok.next();
   }
 }
 
 const char *asciify(int c)