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