void setSchema(Schema schema) {
        if ( this.schema != null)
            throw new IllegalStateException("Cannot add field set '" + name + "' to schema '" + schema.name() +
                                            "' as it is already added to schema '" + this.schema.name() + "'");
        this.schema = schema;
    }
Returns whether this field is index(es), i.e. does indexing: index.
container-search/src/main/java/com/yahoo/search/schema/FieldSet.java