anchor.diff:  { stdenv, fetchpatch, fetchFromGitHub, meson, ninja, pkgconfig, gettext
   , xmlto, docbook_xsl, docbook_xml_dtd_45, libxslt
   , libstemmer, glib, xapian, libxml2, libyaml, gobjectIntrospection
  -, pcre, itstool
  +, pcre, itstool, vala
   }:
   
   stdenv.mkDerivation rec {
  
positive.code:   mesonFlags = [
      "-Dapidocs=false"
      "-Ddocs=false"
    ];
  
    meta = with stdenv.lib; {
  
negative.code:   nativeBuildInputs = [
      meson ninja pkgconfig gettext
      libxslt xmlto docbook_xsl docbook_xml_dtd_45
      gobjectIntrospection itstool vala
    ];
  
    buildInputs = [ libstemmer pcre glib xapian libxml2 libyaml ];
  