A model of a hidden signature with visible subsignature is a -algebra whose restriction to is D, of course also providing functions for all the methods and attributes declared in , thus:

Given a hidden signature , a
**hidden -algebra** M is a (many sorted) -algebra
M such that its restriction to is D.

Similarly, a **hidden equation** is just a -equation, and a **hidden specification** (or
**hidden theory**) is just a set of -equations. What differs sharply from ordinary algebra is
the notion of behavioral satisfaction of an
equation by an algebra, which is our next topic.

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12 October 1996