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2 Why union AND extension?
Union should be used when combining several specifications Spi to one specification Sp = Sp1 + ...+ Spn and there are no hierarchical dependencies between the sub-specifications, that is, one specification needs the definitions of another specification to work.
Union is symmetric.
The sub-specifications in a union communicate by the same name,
same thing principle.
Extension should be used when there is a hierarchical
relationship between the sub-specifications. In an extension
extension Sp0 Sp1 .. Spn,
each sub-specification Spi extends
its predecessor Spi-1, that is, Spi assumes the definitions of
Spi-1 in its own definitions.
In contrast, in a union, Spi has to repeat the definitions of Spi-1 if
it wants to build on them.
Note that, in this case, because of the same name, same thing
principle, the repeated definitions are guaranteed to have the
same interpretation in Spi and Spi-1.
CoFI
Note: M-1 --0.1-- 18 Sep 1997.
Comments to hubert@mpi-sb.mpg.de