The composition of a match and a build (in either order) can often be simplified.
If the match following a build is incompatible, failure is certain. For example,
<verbatim>
  !Foo(Bar(x), y); ?FooBar(z)
</verbatim>
can be replaced with =fail= since the match will never succeed.

If the build and match _are_ compatible (i.e., if they can be unified), 
the sequence can be simplified. For example, 
<verbatim>
  !Foo(Bar(x), y); ?Foo(z1, z2); ...
</verbatim>
can be replaced with
<verbatim>
  !Bar(x); ?z1; !y; ?z2; !Foo(Bar(x), y); ...
</verbatim>
Subsequent transformations can get rid of the construction of the
entire term, if it is not needed.

These ideas are implemented by simple rewrite rules in module

	* [[%SVNSTRATEGOXT%/trunk/StrategoXT/sc/spec/opt/build-match-laws.str][build-match-laws]]

The rules are applied by the [[Stratego simplifier]]. Often these rules are
triggered by inlining (which brings matches and builds together). The results
are simplified by [[constant and copy propagation]] and [[dead variable elimination]].

-- Main.EelcoVisser - 18 Aug 2003