anchor.diff:    enableFeature = enable: feat: "--${if enable then "enable" else "disable"}-${feat}";
   
   
  -  # Create a fixed width string with additional prefix to match required width
  +  # Create a fixed width string with additional prefix to match
  +  # required width.
     fixedWidthString = width: filler: str:
       let
         strw = lib.stringLength str;
  
positive.code:   escapeShellArg = lib.escape (stringToCharacters "\\ ';$`()|<>\t*[]");
  
  
    # replace characters by their substitutes.  This function is equivalent to
    # the `tr' command except that one character can be replace by multiple
    # ones.  e.g.,
    # replaceChars ["<" ">"] ["&lt;" "&gt;"] "<foo>" returns "&lt;foo&gt;".
    replaceChars = builtins.replaceStrings or (
      del: new: s:
      let
  
negative.code: 
    # Determine whether a string has given prefix/suffix.
    hasPrefix = pref: str:
      substring 0 (stringLength pref) str == pref;
    hasSuffix = suff: str:
      let
        lenStr = stringLength str;
        lenSuff = stringLength suff;
      in lenStr >= lenSuff &&
         substring (lenStr - lenSuff) lenStr str == suff;
  
  
    # Convert a string to a list of characters (i.e. singleton strings).
  