Impacted versions: 
9.0x

Steps to reproduce:
1. Go to Accounting, create customer invoice and validate. Check the Invoice-Number (for example: INV/2016/0010)
2.  Go to Point of Sale and select New Session
3. Add any product or just close & confirm the session
4. Close the POS Session (Validate Closing & Post Entries)
5. Create new customer invoice and validate it. Check the Invoice-Number. 

Current behavior:
Gap between the first and the last invoice because of the closed POS-Session. 

Expected behavior:
No Gap between the two invoices like in Version 8. 

I've tested this behaviour on a 8 and a 9 runbot and my own system. There is no gap on 8 but on 9. I think this is not a feature but a bug. I can't find the missing sequence-numbers in any Journals or in the invoice-list. 

The only way to avoid this, is to select another Sales Journal in the POS-Configuration. But then you can't create an invoice from the POS. 

Is it possible to get the "lost" invoice-numbers somewhere? We are not allowed to have gaps in the invoice-numbering here in Germany. 

IrSequence = self.env['ir.sequence']
        val = {
            'name': _('POS Order %s') % values['name'],
            'padding': 4,
            'prefix': "%s/" % values['name'],
            'code': "pos.order",
            'company_id': values.get('company_id', False),
        }
        
        values['sequence_id'] = IrSequence.create(val).id
        
        
        val.update(name=_('POS order line %s') % values['name'], code='pos.order.line')
        IrSequence.create(val)
        return super(PosConfig, self).create(values)
    @api.multi
    def unlink(self):
        for pos_config in self.filtered(lambda pos_config: pos_config.sequence_id):
            pos_config.sequence_id.unlink()
        return super(PosConfig, self).unlink()
    
    @api.multi
    def open_ui(self):