export declare abstract class ExtraField {
}
interface ExtraDict<T extends ExtraField> {
    [key: string]: T | (() => string);
}
export declare class Extras<ExtraT extends ExtraField = ExtraField> implements ExtraDict<ExtraT> {
    [key: string]: ExtraT | (() => string);
    constructor(fields: Record<string, ExtraT>);
    /**
     * Default string representation.
     */
    toString(): string;
}
export {};