public boolean ignoresInput(Expression root) {
            this.root = root;
            convert(root);
            return ignoresInput;
        }
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