Remove the base OID from the given string.

		>>> import snmp_passpersist as snmp
		>>> pp=snmp.PassPersist(".1.3.6.1.3.53.8")
		>>> pp.cut_oid(".1.3.6.1.3.53.8.28.12")
		'28.12'

def cut_oid(self,full_oid):
		"""
		Remove the base OID from the given string.

		>>> import snmp_passpersist as snmp
		>>> pp=snmp.PassPersist(".1.3.6.1.3.53.8")
		>>> pp.cut_oid(".1.3.6.1.3.53.8.28.12")
		'28.12'
		"""
		if not full_oid.startswith(self.base_oid.rstrip('.')):
			return None
		else:
			return full_oid[len(self.base_oid):]