The "ditto" character is " (double-quote); not , (comma) as
indicated in the manpage; and not ' (single-quote) as indicated
in the man.old -style manpage from which the mdoc manpage was derived.
Some other versions of ddb use single-quote for this feature, and this
part of the code may change again in the near future, but the documentation
now affects reality now.
they
jmc