[UML Semantic:001]: Fragment order

The fragment order for an interaction or interaction operand SHALL be defined by the occurrence vertical position along the lifeline. And the order of occurrence in different lines is constrained to:
- Sent event occurrence occurs before the receive event.
- General order before occurrence occurs before the after occurrence.
- Interaction use occurs before any occurrence that follows it along the lifelines it covers.
- The Combined fragment occurs before any occurrence that follows it along the lifelines it covers.
- The operands of a combined fragment are ordered top to bottom.
- Otherwise the order is not defined. Under the situations the order is not defined, we still apply the vertical position rule, and if the occurrences are at similar or equivalent vertical position, we consider the occurrence on the left most lifeline. We also consider the interaction use and combined fragment before any occurrence on a non-covered lifeline but that the fragment graphically extends over.