Jean-Jacques Grunenwald (1911-1982) was a pupil of Dupré, and succeeded him at St. Sulpice in 1973. He composed a LOT of music, 19 works for organ (of which several have multiple movements). His music is not easy to play, but it rewards those who are willing to work for it - lots of accidentals, complex harmonies, etc. This is one of my favorite pieces of his - it is a study in crescendo, modulation, and decrescendo, built around a descending 6-note motif. Only a master composer could take such simple materials and make something good!