The binding of a term variable does not coincide with its binding. That is, in the strategy expresssion
<verbatim>
  {x : ... ; ?Foo(x) ; ... ; !Bar(x) }
</verbatim>
the variable =x= is first introduced in a new scope, some time later it is bound in a match, and at
the end its binding is used in a build.

This entails that a variable is not necessarily bound when it is used in a build. 
Furthermore, a variable may be matched twice. 
The second match only succeeds if it is against the same term that it was bound to the first time.

For these reasons the [[Stratego Compiler]] generates run-time checks on variable matches and builds. 
Often these checks are superfluous since the variable is sure to be bound or unbound at a certain position
in the program. 
An optimization that avoids these checks allows the compiler to generate simpler code, which may also
be better amenable to optimization by the C compiler.

-- Main.EelcoVisser - 11 Jul 2003

The optimization is now included in the compiler and will be available in StrategoRelease093.

-- Main.EelcoVisser - 17 Jul 2003