+
+Usually, this function returns the fill prefix, a string. However,
+before doing this, the function makes a final check (not specially
+mentioned in the following) that a line starting with this prefix
+wouldn't look like the start of a paragraph. Should this happen, the
+function signals the anomaly by returning @code{nil} instead.
+
+In detail, @code{fill-context-prefix} does this:
+
+@enumerate
+@item
+It takes a candidate for the fill prefix from the first line---it
+tries first the function in @code{adaptive-fill-function} (if any),
+then the regular expression @code{adaptive-fill-regexp} (see below).
+The first non-@code{nil} result of these, or the empty string if
+they're both @code{nil}, becomes the first line's candidate.
+@item
+If the paragraph has as yet only one line, the function tests the
+validity of the prefix candidate just found. The function then
+returns the candidate if it's valid, or a string of spaces otherwise.
+(see the description of @code{adaptive-fill-first-line-regexp} below).
+@item
+When the paragraph already has two lines, the function next looks for
+a prefix candidate on the second line, in just the same way it did for
+the first line. If it doesn't find one, it returns @code{nil}.
+@item
+The function now compares the two candidate prefixes heuristically: if
+the non-whitespace characters in the line 2 candidate occur in the
+same order in the line 1 candidate, the function returns the line 2
+candidate. Otherwise, it returns the largest initial substring which
+is common to both candidates (which might be the empty string).
+@end enumerate