@Override
        protected Expression doConvert(Expression exp) {
            if (exp.requiresInput()) {
                return new StatementExpression(new InputExpression(field.getName()), exp);
            } else {
                return exp;
            }
        }
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