Publisher.flatMap+mapError.swift

import Combine

public extension Publisher {
    func flatMap<P: Publisher>(
        _ transformPublisher: @escaping (Self.Output) -> P,
        mapError transformError: @escaping (Self.Failure) -> P.Failure
    ) -> Publishers.FlatMap<P, Publishers.MapError<Self, P.Failure>> {
        return self.mapError(transformError).flatMap(transformPublisher)
    }
}