Oleg wrote: 1. Add u_replypm permission with the following wording: "Can reply to private messages (ignored if user can send private messages)". I think this name is more consistent and the wording is unfortunate but prevents confusion when someone sets "can reply" to never and "can send" to yes.
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"Can reply to private messages <noscript>(ignored if user can send private messages)</noscript>"Oleg wrote: "Can reply to private messages (ignored if user can send private messages)"
And make it appear (with js) only when the option "can send private messages" is set to No or Never.