I added just added next_doc_tab and prev_doc commands.
_command void next_doc_tab() name_info(','VSARG2_READ_ONLY|VSARG2_ICON|VSARG2_MARK|VSARG2_REQUIRES_MDI_EDITORCTL|VSARG2_LINEHEX)
{
int wid=_MDINextDocumentWindow(p_window_id,'N',false);
wid._set_focus();
}
_command void prev_doc_tab() name_info(','VSARG2_READ_ONLY|VSARG2_ICON|VSARG2_MARK|VSARG2_REQUIRES_MDI_EDITORCTL|VSARG2_LINEHEX)
{
int wid=_MDINextDocumentWindow(p_window_id,'P',false);
wid._set_focus();
}