Optional argument HookFunction, shown in the functions described in this module, defines a function that is called when an unknown form occurs where there is to be a valid expression. If HookFunction is equal to none, there is no hook function.
The called hook function is to return a (possibly deep) list of characters. Function expr/4 is useful in a hook.
If CurrentIndentation is negative, there are no line breaks and only a space is used as a separator.