Once the preliminary analysis is done, the optimization phase performs certain transformations on the code. The result is an equivalent program which will (probably) compile into more efficient code. This new program is itself structurally a valid SCHEME program; that is, all transformations are contained within the language. The transformations are thus similar to those performed on macro calls, consisting of a syntactic rewriting of an expression, except that the situations where such transformations are applicable are more easily recognized in the case of macro calls. It should be clear that the optimizer and the macro~functions are conceptually at the same level in that they may be written in the same meta-language that operates on representations of SCHEME programs. {Note Non-deterministic Optimization}
The simplest transformation is that a combination whose function position contains a LAMBDA-expression and which has no arguments can be replaced by the body of the LAMBDA-expression:
((LAMBDA () body)) => body
Another is that, in the case of a LAMBDA-combination, if some parameter of the LAMBDA-expression is not referenced and the corresponding argument can be proved to have no side-effects (with an exception discussed below), then the parameter and argument can be eliminated:
((LAMBDA (x1 x2 x3) body) a1 a2 a3)
=> ((LAMBDA (x1 x3) body) a1 a3)
if x2 is unreferenced in body and a2 has no side-effects
Repeated applications of this rule can lead to the preceding case.
A third rule is that, in a LAMBDA-combination, an argument can be
