modify custom variables

<div class="moz-text-flowed" lang="x-western" style="font-family: -moz-fixed;     font-size: 12px;">Hi, <br /><br />
Whencreating postgis extension, a call to DefineCustomStringVariable is made to define postgis.backend.<br /><br /> An
assignhook allows to change the library used by spatial functions whith a:<br /><br />     SET postgis.backend =
'sfcgal';-- or 'geos' <br /><br /> Everything work fine execept when upgrading postgis.<br /><br /> If I understand the
problemcorrectly, on upgrate (ALTER EXTENSION), the old postgis library is already loaded, the old spatial functions
arereplaced by new ones pointing to the new postgis library, a call to DefineCustomStringVariable is made and:<br /><br
/>    ERROR:  attempt to redefine parameter "postgis.backend"<br /><br /> I need to redefine the assign hook such that
iscalls the new library backend and not the old one.<br /><br /> Is there a way to remove/modify custom variables ? I
lookedin guc.h but couldn't find anything.<br /><br /> Thanks, <br /><br /> V. <br /><br /><br /><br /> PS: I'm working
on<a class="moz-txt-link-freetext"
href="http://trac.osgeo.org/postgis/ticket/2382">http://trac.osgeo.org/postgis/ticket/2382</a>.<br /></div>