anchor.diff:  
     src = fetchurl rec {
       url = "http://www.fftw.org/fftw-${version}.tar.gz";
  -    md5Url = url + ".md5sum";
  +    multihash = "QmPVvMpw6XJhgiEv6X8vPPA2xLaGdjuUvQrGg5ASeKpeMy";
  +    hashOutput = false;
       sha256 = "8ecfe1b04732ec3f5b7d279fdb8efcad536d555f9d1e8fabd027037d45ea8bcf";
     };
   
     configureFlags = [
  -    (enFlag "single" (precision == "single") null)
  -    (enFlag "float" (precision == "single") null)
  -    ###(enFlag "double" (precision == "double") null)
  -    (enFlag "long-double" (precision == "long-double") null)
  -    (enFlag "quad-precision" (precision == "quad-precision") null)
  -    (enFlag "sse" (
  +    "--${boolEn (precision == "single")}-single"
  +    "--${boolEn (precision == "single")}-float"
  +    ###"--${boolEn (precision == "double")}-double"
  +    "--${boolEn (precision == "long-double")}-long-double"
  +    "--${boolEn (precision == "quad-precision")}-quad-precision"
  +    "--${boolEn (
         elem targetSystem platforms.x86-all
  -      && precision == "single") null)
  -    (enFlag "sse2" (
  +      && precision == "single")}-sse"
  +    "--${boolEn (
         elem targetSystem platforms.x86-all
  -      && (precision == "single" || precision == "double")) null)
  +      && (precision == "single" || precision == "double"))}-sse2"
       # Could be enabled when our minimum is sandy bridge
       "--disable-avx"
       # Could be enabled when our minimum is haswell
  
positive.code:     "-fPIC"
    ];
  
    meta = with stdenv.lib; {
      description = "Library for Fast Discrete Fourier Transform";
      homepage = http://www.fftw.org/;
      license = licenses.gpl2Plus;
  
negative.code: { stdenv
  , fetchurl
  , lib
  
  , precision
  }:
  