[Groff] How remove title?

[Peng Yu]

Hi, I use the following command to generate a pdf file from a manpage.
But the pdf file has title "Untitled". I want to unset it via groff.
But I don't see an option to set title to be null. Would you please
let me know how to unset the title with groff?

groff -Tps -mandoc -c < $(man -w ls) | ps2pdf - > /tmp/ls.pdf


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Regards,
Peng