enable or disable history manager
state1=historymanager(state2)
state1=historymanager()
:state1 returns history manager state ‘on’ or ‘off’ : :state2 ‘on’ or ‘off’ set history manager’s state :
`displayhistory`_()
backupstate=historymanager()
historymanager('off')
`displayhistory`_()
historymanager('on')
`loadhistory`_()
`displayhistory`_()
historymanager(backupstate)