Please annotate with reference to context the following line: "Alexander died, Alexander was buried, Alexander returneth to dust."

This quote is from Hamlet Act 5, Scene I.  Hamlet is in the process of returning to Elsinore and he and Horatio happen across gravediggers preparing a grave.  After some banter, the scene progresses to Hamlet's handling of the long-buried skull of Yorick, a jester Hamlet knew well and was very fond of in his youth.  Hamlet is pondering the state of life and death and what existence leads to.  The paragraph that includes the quote follows:

No, faith, not a jot [he is not being too serious to think like this], but to follow him thither with modesty enough, and likelihood to lead it.  Alexander died, Alexander was buried.  Alexander returneth to dust; the dust is earth; of earth we make loam; and what of that loam whereto he was converted might they not stop a beer-barrel?

Hamlet then says much the same using Caesar as an example:  Caesar's dust might ultimately be used to plug a hole to stop the wind.

This, suggests Hamlet, is what life comes to.  Shakespeare here continues the imagery he started when Hamlet jokes to Claudius about Polonius' body being in a place where he is eaten, not where he eats, etc., in Act IV, Scene 3.