code: func (ps *PrintSettings) SetLength(key string, value float64, unit Unit) {
  	cstr := C.CString(key)
  	defer C.free(unsafe.Pointer(cstr))
  	C.gtk_print_settings_set_length(ps.native(), (*C.gchar)(cstr), C.gdouble(value), C.GtkUnit(unit))
  }
docs: // SetLength() is a wrapper around gtk_print_settings_set_length().