An alist associating X Windows selection-types with functions.
These functions are called to convert the selection, with three args:
the name of the selection (typically `PRIMARY', `SECONDARY', or `CLIPBOARD');
a desired type to which the selection should be converted;
and the local selection value (whatever was given to `x-own-selection').
The function should return the value to send to the X server
(typically a string). A return value of nil
means that the conversion could not be done.
A return value which is the symbol `NULL'
means that a side-effect was executed,
and there is no meaningful selection value.