if expression just discussed is the else-part of an enclosing
if expression which tests whether there is a fill prefix. If
there is a fill prefix, the then-part of this
if is evaluated.
It looks like this:
(while (and (not (eobp)) (not (looking-at paragraph-separate)) (looking-at fill-prefix-regexp)) (forward-line 1))
What this expression does is move point forward line by line so long as three conditions are true:
The last condition may be puzzling, until you remember that point was
moved to the beginning of the line early in the
function. This means that if the text has a fill prefix, the
looking-at function will see it.