+- mandoc replaces all ASCII control characters except tab and line feed
+ with '?' during input. It would be better to replace them with
+ Unicode escapes in preconv_encode() or somewhere in the vicinity,
+ such that the already existing better replacement strings show
+ up in the output. Emulating groff is not desirable: groff replaces
+ 0x00, 0x0b, and 0x0d to 0x1f with the empty string (bad because
+ that's easy to overlook for the document author), 0x01 with '.'
+ (very confusing), and passes through 0x02 to 0x08, 0x0c, and 0x7f
+ raw (bad because that is insecure output). Remember that 0x07 may
+ need special handling because it is sometimes used for certain
+ delimiters, so it may need handling *after* roff.c rather than before.
+ reminded by John Gardner 16 Jun 2020 14:26:28 +1000
+ Actually, more ASCII control characters than just 0x07 may need
+ later handling because they can for example be used in macro names.
+ So they may need handling after roff(7) processing.
+ pointed out by John Gardner 23 Jun 2020 18:28:08 +1000
+ more info from John Gardner 29 Jun 2020 19:54:04 +1000
+ loc ** exist ** algo ** size ** imp *
+
+- many missing features used in old groff_char(7),
+ some can possibly be supported
+ kamil at netbsd 12 Nov 2020 17:27:09 +0100 + reply
+
+- \s with arbitrary arg delimiters as already supported for other escapes
+ found following jmc@'s mail 28 Apr 2021 18:31:41 +0100
+ loc * exist * algo * size * imp *
+