www/src/py_string.js
function replace_nested(name, key){
                if(/\d+/.exec(key)){
                    // If key is numeric, search in positional
                    // arguments
                    return _b_.tuple.__getitem__($.$args,
                        parseInt(key))
                }else{
                    // Else try in keyword arguments
                    return _b_.dict.__getitem__($.$kw, key)
                }
            }
function replace_nested(name, key){
                if(/\d+/.exec(key)){
                    // If key is numeric, search in positional
                    // arguments
                    return _b_.tuple.__getitem__($.$args,
                        parseInt(key))
                }else{
                    // Else try in keyword arguments
                    return _b_.dict.__getitem__($.$kw, key)
                }
            }
"spec" may contain "nested replacement fields" In this case, evaluate them using the positional or keyword arguments passed to format()