I have a small question regarding the usage of notation in this book. It is defining differential forms and their properties in its section, but the $\mid$ operator is being used in a way that I haven't seen before. Of course, I checked their list of notations at the beginning of the book, but I didn't see it there. I'm only interested in item (iv), but I'll give you the rest of the items and the theorem statement so you can see what is going on contextually. For those who own the book, the items are listed on page 106:
What does the "$\mid$" in "$d(\alpha \mid \mathfrak U) = (d\alpha) \mid \mathfrak U$" mean here? Obviously I've seen the glyph used before, but not in this context. Cheers and thanks in advance!