Chiasmus

The simplest example would be something like "I love you as you love me."

Slightly more elaborate in form: instead of the straightforward: I cannot dig; I am ashamed to beg, use of chiasmus might produce: I cannot dig; to beg I am ashamed.

The term is typically used to refer only to more elaborate forms, such as Frederick Douglass, "You have seen how a man became a slave; you shall see how a slave becomes a man."

Perhaps the most famous example is John F. Kennedy's "Ask not what your country can do for you -- ask what you can do for your country."