I am sending out a Hail Mary here. Louis Couperin (1626–1661) wrote a fugue on precisely the same theme as his slightly older colleage, Jean-Henry D'Anglebert. In D'Anglebert's publication of five fugues, which we believe were on a plainsong melody. Does anyone recognise the chant, by any chance. The background is that Couperin was organist at Saint-Gervais, Paris, after 1653, though I find nothing in the usual antiphoners and graduals of that period.
I was looking at D'Anglebert this Fall and, thinking it would make a nice program with some of Die Kunst, thought of adding Louis Couperin's phrygian fugues as well as Couperin the Tall's Cunctipotens fugue. I should have looked further! Why would one assume the subject was plainsong, though?
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