In this arrangement of Wachet Auf, there are quite a few mordents sprinkled around, as well as trills and grace notes. I understand most of them, but in the middle of this excerpt,
right before the bar line, there is a mordent that appears to the left of the F, instead of over it. Does it mean the same as a mordent over a note?
And what about the slur? That seems to suggest that the mordent is actually played below the F. Here is a magnified view of the mordent and slur in question:
Update and state of the question:
It's been argued convincingly by guidot and alephzero that the "slur" is not a slur, but is actually an attempt to graphically reproduce (with modern, more automated tools) the notation shown in this older, hand-engraved manuscript:
But the question remains, what does this notation mean? Including, how should it be realized? I don't think it's a doppel cadence (reversed or otherwise) from Bach's guide to ornaments, since it appears to the left of, and below, the principal note.
Meyer and Patrx say that it's a slide (two grace notes leading up to the principal), which makes a lot of sense in the context. I haven't yet seen a source (like Lovelock) where a slide is documented as being written with a tail, but the realization is confirmed by recordings like this one (0:24) and others.