mail-yank-region
mail-yank-region is an interactive compiled Lisp function in `sendmail.el
'.
(mail-yank-region ARG)
Insert the selected region from the message being replied to.
Puts point after the text and mark before.
Normally, indents each nonblank line ARG spaces (default 3).
However, if `mail-yank-prefix' is non-nil, insert that prefix on each line.
Just C-u as argument means don't indent, insert no prefix,
and don't delete any header fields.