code: func (ps *PrintSettings) SetSefaultSource(defaultSource string) {
  	cstr := C.CString(defaultSource)
  	defer C.free(unsafe.Pointer(cstr))
  	C.gtk_print_settings_set_default_source(ps.native(), (*C.gchar)(cstr))
  }
docs: // SetSefaultSource() is a wrapper around gtk_print_settings_set_default_source().