When Drawing a Sequence Diagram, the Analyst Uses Lifelines and a Symbol

When drawing a sequence diagram, the analyst uses lifelines and a symbol to represent the execution occurrence. It is also possible for an object to be destroyed at some point in time. Explain these three concepts - what is a lifeline, what does the execution occurrence mean, and how it is possible that an object can be destroyed in a use case. Show the diagramming symbols used to represent each of these concepts.