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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.