@Override
    protected boolean shouldConvert(Expression exp) {
        return exp instanceof StatementExpression;
    }
Optimizes expressions by removing expressions that have no effect.
Typical examples are statements that come before a statement that
generates a new execution value without regard for the existing one.
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/ExpressionOptimizer.java