fill-context-prefix

fill-context-prefix is a compiled Lisp function in `fill.el'.

(fill-context-prefix FROM TO &optional FIRST-LINE-REGEXP)

Compute a fill prefix from the text between FROM and TO.
This uses the variables `adaptive-fill-regexp' and `adaptive-fill-function'
and `adaptive-fill-first-line-regexp'. `paragraph-start' also plays a role;
we reject a prefix based on a one-line paragraph if that prefix would
act as a paragraph-separator.