anchor.diff:  { stdenv, fetchurl, pkgconfig, openssl, libxslt, perl
  -, curl, pcre, libxml2, librdf_rasqal
  +, curl, pcre, libxml2, librdf_rasqal, gmp
   , mysql, withMysql ? false
   , postgresql, withPostgresql ? false
   , sqlite, withSqlite ? true
  
positive.code: 
    meta = {
      homepage = http://librdf.org/;
      platforms = stdenv.lib.platforms.linux;
    };
  }
  
negative.code: 
    nativeBuildInputs = [ perl pkgconfig ];
  
    buildInputs = [ openssl libxslt curl pcre libxml2 gmp ]
      ++ stdenv.lib.optional withMysql mysql.connector-c
      ++ stdenv.lib.optional withSqlite sqlite
      ++ stdenv.lib.optional withPostgresql postgresql
  