anchor.diff:      , text
       , executable ? false # run chmod +x ?
       , destination ? ""   # relative path appended to $out eg "/bin/foo"
  +    , checkPhase ? ""    # syntax checks, e.g. for scripts
       }:
       runCommand name
         { inherit text executable;
  
positive.code:   writeScript = name: text: writeTextFile {inherit name text; executable = true;};
    writeScriptBin = name: text: writeTextFile {inherit name text; executable = true; destination = "/bin/${name}";};
  
  
    # Create a forest of symlinks to the files in `paths'.
    symlinkJoin =
  
negative.code:           echo -n "$text" > "$n"
          fi
  
          ${checkPhase}
  
          (test -n "$executable" && chmod +x "$n") || true
        '';
  
  