Forwarding I/O activity

An indirectbuf is one that forwards all of its I/O requests to another streambuf.

An indirectbuf can be used to implement Common Lisp synonym-streams and two-way-streams:

class synonymbuf : public indirectbuf {
   Symbol *sym;
   synonymbuf(Symbol *s) { sym = s; }
   virtual streambuf *lookup_stream(int mode) {
       return coerce_to_streambuf(lookup_value(sym)); }
};