The inimitable Fr Z has provided an excellent podcazt about the Sequence Veni Sancte Spiritus which is sung throughout Pentecost and its Octave; he points out what I had not previously noted - the chiastic structure of the Sequence, which I will now summarize:
1. & 2. Veni... Veni... Veni... Veni... (once in the former, thrice in the latter stanza)3. & 4. Six titles of the Holy Ghost (three in each stanza)5. & 6. Transition and summary7. & 8. Six petitions to the Holy Ghost (three in each stanza)9. & 10. Da... Da... Da... Da... (once in the former, thrice in the latter stanza)
In each pair of stanzas, the same tune is repeated, of course, so grouping them into five strophes. The Sequence many times contrasts positives and negatives: the presence of the Spirit, or His absence; what He does in us, and what we are without Him; fulness and emptiness.
(I had myself previously noted how this Sequence expands upon the words of the preceding Alleluia verse, Veni, Sancte Spiritus, reple tuorum corda fidelium... and in fact repeats many of its words: Veni, Sancte Spiritus... Veni... Veni... Veni... cordium... Reple cordis... Tuorum fidelium...)
The last pair of stanzas is particularly important, in that in the first of the two we ask for the sevenfold gifts of the Holy Spirit, which we need in this present life, while in the second we ask for the reward of virtue, a happy end, and eternal joy - that is, for eternal life. I recall Pope John Paul II in one of his writings repeated this closing line:
Da virtutis meritum,
Da salutis exitum,
Da perenne gaudium.