Actions in a sequence diagrams may also be started by actors who are ideally represented on the left most side of the sequence diagram you can use [ ] to represent actors like

[Bob]->PizzaRequestForm:Submits
[Bob]->PizzaCancelForm:Submits

the resultant diagram should have figures of actor named Bob on the left with an arrow for Submits to the PizzaRequestForm & PizzaCancelForm respectively one below the other