Choice operator. Try left; if it fails, try right.

This is a problematic combinator:

• Implementation is clumsy due to internal parser state being untouchable. We must record seen characters, in order to replay them on the right parser in case the left parser fails.
• Errors degrade due to left parser errors being discarded. Perhaps your string was one character off a successful left parse; but if it fails, you won't see that error.
• It's hard to reason aobut. It might break in certain situations.