@Override
    protected Expression doConvert(Expression exp) {
        return optimizeStatement((StatementExpression) exp);
    }
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