-The filename can be adjusted with the &'tag'& option, which
-may access any variables already defined. The logging may be adjusted with
-the &'opts'& option, which takes the same values as the &`-d`& command-line
-option.
-Logging started this way may be stopped, and the file removed,
-with the &'kill'& option.
+
+.new
+Options are a slash-separated list.
+If an option takes an argument, the option name and argument are separated by
+an equals character.
+Several options are supported:
+.wen
+.display
+tag=<&'suffix'&> The filename can be adjusted with thise option.
+ The argument, which may access any variables already defined,
+ is appended to the default name.
+
+opts=<&'debug&~options'&> The argument specififes what is to be logged,
+ using the same values as the &`-d`& command-line option.
+
+stop Logging started with this control may be
+ stopped by using this option.
+
+kill Logging started with this control may be
+ stopped by using this option.
+ Additionally the debug file will be removed,
+ providing one means for speculative debug tracing.
+
+pretrigger=<&'size'&> This option specifies a memory buffuer to be used
+ for pre-trigger debug capture.
+ Debug lines are recorded in the buffer until
+ and if) a trigger occurs; at which time they are
+ dumped to the debug file. Newer lines displace the
+ oldest if the buffer is full. After a trigger,
+ immediate writes to file are done as normal.
+
+trigger=<&'reason'&> This option selects cause for the pretrigger buffer
+ see above) to be copied to file. A reason of $*now*
+ take effect immediately; one of &*paniclog*& triggers
+ on a write to the panic log.
+.endd
+