@Override
        protected void doVisit(Expression exp) {
            if ( ! (exp instanceof InputExpression)) return;
            var referencedFieldName = ((InputExpression)exp).getFieldName();
            var referencedField = schema.getField(referencedFieldName);
            if (referencedField == null || ! referencedField.hasFullIndexingDocprocRights())
                fail(schema, field, "Indexing script refers to field '" + referencedFieldName +
                                    "' which is neither a field in " + schema.getDocument() + " nor a mutable attribute");
        }
This processor modifies all indexing scripts so that they input the value of the owning field by default. It also
ensures that all fields used as input exist.

@author Simon Thoresen Hult
config-model/src/main/java/com/yahoo/schema/processing/IndexingInputs.java