# The Simple Language

The simple language provides various elementary expressions that return different parts of a message exchange. For example, the expression, `header.timeOfDay`, would return the contents of a header called `timeOfDay` from the incoming message. You can also construct predicates by testing expressions for equality. For example, the predicate, `${header.timeOfDay} == '14:30'`, tests whether the `timeOfDay` header in the incoming message is equal to `14:30`.