@Override
        protected void doVisit(Expression exp) {
            if (exp instanceof OutputExpression) {
                touchedFields.add(((OutputExpression) exp).getFieldName());
            }
            if (exp instanceof InputExpression) {
                candidates.add(((InputExpression) exp).getFieldName());
            }
            if (exp instanceof ZCurveExpression) {
                touchedFields.addAll(candidates);
            }
        }
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