anchor.diff:      "https://mirror.chpc.utah.edu/pub/vault.centos.org/"
       "https://mirror.math.princeton.edu/pub/centos-vault/"
       "https://mirrors.tripadvisor.com/centos-vault/"
  +    "http://mirror.centos.org/centos/"
     ];
   
     # Debian.
     debian = [
       "https://httpredir.debian.org/debian/"
  +    "https://ftp.debian.org/debian/"
       "ftp://ftp.de.debian.org/debian/"
       "ftp://ftp.fr.debian.org/debian/"
       "ftp://ftp.nl.debian.org/debian/"
       "ftp://ftp.ru.debian.org/debian/"
  -    "https://ftp.debian.org/debian/"
       "http://archive.debian.org/debian-archive/debian/"
       "ftp://ftp.funet.fi/pub/mirrors/ftp.debian.org/debian/"
     ];
  
positive.code: 
    # Sage mirrors (https://www.sagemath.org/mirrors.html)
    sageupstream = [
      # Africa
      "ftp://ftp.sun.ac.za/pub/mirrors/www.sagemath.org/spkg/upstream/"
      "https://sagemath.mirror.ac.za/spkg/upstream/"
      "https://mirror.ufs.ac.za/sagemath/spkg/upstream/"
  
      # America, North
      "https://mirrors.mit.edu/sage/spkg/upstream/"
      "http://www.cecm.sfu.ca/sage/spkg/upstream/"
      "https://mirrors.xmission.com/sage/spkg/upstream/"
  
      # America, South
      "http://sagemath.c3sl.ufpr.br/spkg/upstream/"
      "http://linorg.usp.br/sage/spkg/upstream"
  
      # Asia
      "http://ftp.kaist.ac.kr/sage/spkg/upstream/"
      "http://ftp.riken.jp/sagemath/spkg/upstream/"
      "https://mirrors.tuna.tsinghua.edu.cn/sagemath/spkg/upstream/"
      "https://mirrors.ustc.edu.cn/sagemath/spkg/upstream/"
      "http://ftp.tsukuba.wide.ad.jp/software/sage/spkg/upstream/"
      "https://ftp.yz.yamagata-u.ac.jp/pub/math/sage/spkg/upstream/"
      "https://mirror.yandex.ru/mirrors/sage.math.washington.edu/spkg/upstream/"
  
      # Australia
      "https://mirror.aarnet.edu.au/pub/sage/spkg/upstream/"
  
      # Europe
      "https://sage.mirror.garr.it/mirrors/sage/spkg/upstream/"
      "http://mirrors.fe.up.pt/pub/sage/spkg/upstream/"
      "https://www-ftp.lip6.fr/pub/math/sagemath/spkg/upstream/"
      "http://ftp.ntua.gr/pub/sagemath/spkg/upstream/"
    ];
  
  
negative.code:   fedora = [
      "https://archives.fedoraproject.org/pub/fedora/"
      "https://fedora.osuosl.org/"
      "https://ftp.funet.fi/pub/mirrors/ftp.redhat.com/pub/fedora/"
      "https://ftp.linux.cz/pub/linux/fedora/"
      "https://archives.fedoraproject.org/pub/archive/fedora/"
      "http://ftp.nluug.nl/pub/os/Linux/distr/fedora/"
      "http://mirror.csclub.uwaterloo.ca/fedora/"
      "http://mirror.1000mbps.com/fedora/"
    ];
  
    # openSUSE.
  