extern Canvas::Handle open_canvas(const String &filename,String &errors,String &warnings);
extern Canvas::Handle open_canvas_as(const String &filename,const String &as,String &errors,String &warnings);
extern Canvas::Handle open_canvas(const String &filename,String &errors,String &warnings);
extern Canvas::Handle open_canvas_as(const String &filename,const String &as,String &errors,String &warnings);