Summary: re'u is semantically related to the ZAhO series, in that it specifies a phase of the event. (This discussion is suggestive of calling re'u an aspect, and not a tense.)
Summary: za'u re'u means "not for the first time", so it could be used for some senses of "again".
roi
http://www.lojban.org/lists/jboske/msg00218.html
Summary: NB! ROI is a sumti tcita, too. The sumti specifies which time interval the repetitions occur. Quantification issues! Uncertain of how this works with xorlo, taking the discussion to jboske.
Summary: no CLL examples of re'u/roi with tagged sumti!! Can the meaning of the tagged sumti be understood in relation to that of TAhE (for roi) and ZAhO (for re'u)?