@Override
        protected void doVisit(Expression expression) {
            if (modifiesSelf()) return;

            if (expression instanceof InputExpression) {
                inputField = ((InputExpression) expression).getFieldName();
            }
            if (expression instanceof OutputExpression) {
                outputField = ((OutputExpression) expression).getFieldName();
            }
        }
An indexing language script derived from a schema. An indexing script contains a set of indexing
statements, organized in a composite structure of indexing code snippets.

@author bratseth
config-model/src/main/java/com/yahoo/schema/derived/IndexingScript.java