Node:fwd-para within paragraph, Next:fwd-para no fill prefix, Previous:fwd-para between paragraphs, Up:forward-paragraph
The next expression in the outer while loop is an if
expression. The Lisp interpreter evaluates the then-part of the
if when the fill-prefix-regexp variable has a value other
than nil, and it evaluates the else-part when the value of
if fill-prefix-regexp is nil, that is, when there is no
fill prefix.