@Override
        protected boolean shouldConvert(Expression exp) {
            return exp instanceof StatementExpression
                   && ( field.isDocumentField() || ( field.getAttribute() != null && field.getAttribute().isMutable()));
        }
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