Here's what I found:
A. I cut back via UNDO until I could get the PDF to format correctly.

## Advertising

B. Then I hit REDO once.
C. I saw that the last UNDO eliminated an equation that had the [PARAGRAPH]
identifier beside it (on LHS).
D. I thought: Problem solved. But question why it occurred? I often cut some
math from a text line and paste it into an equation; eg, when I introduce math
terms in the paragraph before the equation.
E: So I hit UNDO again; checked PDF formats OK; then began entering new text,
etc.
F: 20 minutes later, PDF will not format.
G: Repeated procedure from A-C above and found that I had to UNDO to an OLDER
equation with [PARAGRAPH] identifier beside it (on LHS).
H. So did E again. Problem is now fixed!
I. Interesting behaviour: that the older PARAGRAPH] [EQUATION] took time to
trigger?
J. Working OK now, EXCEPT: Maybe my latest BUG report is related; FIND and
CHANGE in MATH mode triggers BUG-report. Huge report (too big to copy) sent to
Apple via the Mac system; so I hope it can be recovered from them. I was
putting hats ^ on some vectors.