An informal description can be used escape the rigor of formal specification. The Uses Clause for how abstract values can be informally described.
Informal descriptions have the following syntax.
For example, the following specification uses an informal precondition, and an informal postcondition. If the find yourself describing some activity, you should rephrase your statement to describe the result of that activity. That is, one can write completely informal statements about the behavior of a function, later make some parts of the informal statements formal, and still later refine these into completely formal statements. For example, the following specification has a completely formal precondition, and a partly informal postcondition. See section 10.2 Class Refinement for an example using informal descriptions.