anchor.diff:      */
     toYAML = {}@args: toJSON args;
   
  +
     /* Pretty print a value, akin to `builtins.trace`.
       * Should probably be a builtin as well.
       */
  
positive.code:               (name: value:
                  "${toPretty args name} = ${toPretty args value};") v)
          + " }"
      else abort "toPretty: should never happen (v = ${v})";
  
  }
  
negative.code:     allowPrettyValues ? false
    }@args: v: with builtins;
      if      isInt      v then toString v
      else if isString   v then ''"${libStr.escape [''"''] v}"''
      else if true  ==   v then "true"
      else if false ==   v then "false"
      else if null ==    v then "null"
      else if isFunction v then
        let fna = lib.functionArgs v;
  