anchor.diff:  note right #lightgray: payment method\nnot necessarily\nsame as that\nof invoice
   
   
  -IncomingInvoice <--- "0..* " Payment
  +
  +
  +
  +IncomingInvoice "payment" <--- "0..* " Payment
  +note left #lightgray: can only refer to invoices,\nnot credit notes.\nif invoice has credit notes,\nthen we also derive\nthat the payment pays off\na those credit notes also
  +
  +
   
   PaymentMethod <--- Payment
   
  
positive.code:     project: Project
  }
  
  Order <--> "0..*" OrderItem
  
  
  Party "buyer" <-- Order
  Party "seller" <-- Order
  
  
  
  
  
negative.code: class OrderItemInvoiceItemLink #pink {
  
  }
  
  
  ' some invoices will have credit note(s) pointing back to them
  IncomingInvoice "0..1" <-- "credit note" IncomingInvoice
  
  
  IncomingInvoiceItem <--- OrderItemInvoiceItemLink
  OrderItem <--- OrderItemInvoiceItemLink
  