It seems to be that the `:html-preamble` and `:html-postamble` export
options are impotent. They are overridden if the
`org-export-html-preamble-format` and `org-export-html-postamble-format`
are set to *anything*.
In order for the `:html-preamble` and `:html-postamble` html export
options to even be considered during export, one must set their
corresponding format variables to `nil`. But by then there's effectively
no preamble or postamble to disable anyway.
How is one expected to make use of the :html-preamble` and
`:html-postamble` options? (Perhaps I have misunderstood something).
In any case, shouldn't the options override the internal variables
during export on the per-file basis?
Thank you for your time,
Luca