mail-header-merge

mail-header-merge is a compiled Lisp function in `mailheader.el'.

(mail-header-merge MERGE-RULES HEADERS)

Return a new header alist with MERGE-RULES applied to HEADERS.
MERGE-RULES is an alist whose keys are header names (symbols) and whose
values are forms to evaluate, the results of which are the new headers. It
should be a string or a list of string. The first element may be nil to
denote that the formatting functions must use the remaining elements, or
skip the header altogether if there are no other elements.
The macro `mail-header' can be used to access headers in HEADERS.