+unbound. If the message has a plural form, environment variable
+<CODE>MSGFILTER_MSGID_PLURAL</CODE> is bound to the message's msgid_plural and
+<CODE>MSGFILTER_PLURAL_FORM</CODE> is bound to the order number of the plural
+actually processed (starting with 0), otherwise both are unbound.
+If the message has a previous msgid (added by <CODE>msgmerge</CODE>),
+environment variable <CODE>MSGFILTER_PREV_MSGCTXT</CODE> is bound to the
+message's previous msgctxt, <CODE>MSGFILTER_PREV_MSGID</CODE> is bound to
+the previous msgid, and <CODE>MSGFILTER_PREV_MSGID_PLURAL</CODE> is bound to
+the previous msgid_plural.