Reply
type Reply = [Context, Result]
The type representing a reply from a parser. Each parser returns a value of this type, and parse does as well.
Reply objects should be produced by okReply, failReply, and fatalReply rather than being produced by hand. This ensures that the context is copied so that there aren't accidental overwrites of the indexes of older contexts.
A reply contains all of the information necessary to get a result and, if that result is a failure, to know what the failure was and where it happened.