Define Mule Variations Meaning

Mule Variations
This album was Tom Waits' 1999 release, and his thirteenth. The songs range from Big in Japan, a smooth but tough rock song, to Come On Up To the House, a powerful telling of a world's troubles. Waits did good again, striking a strange chord with Chocolate Jesus, and tugging your thought strings with Black Market Baby.

An excerpt from Mule Variations'-
Get Behind the Mule

Punctuated birds on the power line
In a Studebaker with the Birdie Joe Joaks
I'm diggin' all the way to China
With a silver spoon
While the hangman fumbles with the noose, boy
The hangman fumbles with the noose
Got to get behind the Mule
In the morning and plow
By Bobina