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(emacs)Adaptive Fill

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Adaptive Filling
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   The fill commands can deduce the proper fill prefix for a paragraph
automatically in certain cases: either whitespace or certain punctuation
characters at the beginning of a line are propagated to all lines of the
paragraph.

   If the paragraph has two or more lines, the fill prefix is taken from
the paragraph's second line, but only if it appears on the first line as
well.

   If a paragraph has just one line, fill commands _may_ take a prefix
from that line.  The decision is complicated because there are three
reasonable things to do in such a case:

   * Use the first line's prefix on all the lines of the paragraph.

   * Indent subsequent lines with whitespace, so that they line up
     under the text that follows the prefix on the first line, but
     don't actually copy the prefix from the first line.

   * Don't do anything special with the second and following lines.

   All three of these styles of formatting are commonly used.  So the
fill commands try to determine what you would like, based on the prefix
that appears and on the major mode.  Here is how.

   If the prefix found on the first line matches
`adaptive-fill-first-line-regexp', or if it appears to be a
comment-starting sequence (this depends on the major mode), then the
prefix found is used for filling the paragraph, provided it would not
act as a paragraph starter on subsequent lines.

   Otherwise, the prefix found is converted to an equivalent number of
spaces, and those spaces are used as the fill prefix for the rest of the
lines, provided they would not act as a paragraph starter on subsequent
lines.

   In Text mode, and other modes where only blank lines and page
delimiters separate paragraphs, the prefix chosen by adaptive filling
never acts as a paragraph starter, so it can always be used for filling.

   The variable `adaptive-fill-regexp' determines what kinds of line
beginnings can serve as a fill prefix: any characters at the start of
the line that match this regular expression are used.  If you set the
variable `adaptive-fill-mode' to `nil', the fill prefix is never chosen
automatically.

   You can specify more complex ways of choosing a fill prefix
automatically by setting the variable `adaptive-fill-function' to a
function.  This function is called with point after the left margin of a
line, and it should return the appropriate fill prefix based on that
line.  If it returns `nil', that means it sees no fill prefix in that
line.