postinst is in gpr 0.15deb-2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
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#log_failure_msg () {
#        echo "$@"
#}
#[ -r /lib/lsb/init-functions ] && source  /lib/lsb/init-functions
W=/usr/lib/gpr/lpr.wrapper
D=/usr/bin/lpr.not.gpr
L=/usr/bin/lpr
if [ "$1" = reconfigure ] || [ "$1" = configure ] ; then
  . /usr/share/debconf/confmodule
  db_get gpr/divert_lpr 
  db_stop
  if [ "$RET" = "true" ]; then
####### install lpr wrapper
      #add diversion
      if dpkg-divert --list gpr | grep -q $D ; then : ; else
	  dpkg-divert --package gpr --add --rename \
	              --divert $D $L
      fi
      #check (and create) link
      if [ ! -e $L ] ; then
	  ln -s $W $L 	  
      elif [ ! -L $L ] ; then
	  echo gpr problem: $L exists and is not a symlink 1>&2
	  exit 1
      elif find $L -printf '%l' | grep -q $W ; then
	  true
      elif find $L -printf '%l' | grep -q $D ; then
	  #legacy from older versions
	  rm $L
	  ln -s $W $L 	  
      else
	  echo gpr problem: symlink $L does not point to $W 1>&2
	  exit 2
      fi
####### deinstall lpr wrapper
  else
    #act only if diversion is there
    if  dpkg-divert --list gpr | grep -q $D ; then
        #remove the link?
	if [  -L $L ] && find $L -printf '%l' | egrep -q "$W|$D" ; then
		rm $L
	fi
        #remove the diversion?
	if [ -e  $L ] ; then 
	    echo gpr problem: $L exists, cannot remove diversion 1>&2
	    exit 5
	else
	    dpkg-divert --package gpr --remove --rename \
		--divert $D $L 
	fi
    fi
  fi
fi
# Automatically added by dh_installmenu
if [ "$1" = "configure" ] && [ -x "`which update-menus 2>/dev/null`" ]; then
	update-menus
fi
# End automatically added section
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