Provide explanation why batch functions are implemented as macros

Provide explanation why batch functions are implemented as macros

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| The functions `org-batch-agenda', `org-batch-agenda-csv', and
| `org-batch-store-agenda-views' are implemented as macros to provide
| a conveniant way for extracting agenda information from the command
| line. The Lisp does not evaluate parameters of a macro call; thus
| it is not necessary to quote the parameters passed to one of those
| functions. E.g. you can write:
|
|   emacs -batch -l ~/.emacs -eval '(org-batch-agenda "a" org-agenda-span 7)'
|
| To export an agenda spanning 7 days. If `org-batch-agenda' would
| have been implemented as a regular function you'd have to quote the
| symbol org-agenda-span. Moreover: To use a symbol as parameter
| value you would have to double quote the symbol.
|
| This is a hack, but it works even when running Org byte-compiled.
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