m.bind(link =>
   !link.has(dst) && compare(src, dst)
     ? (
       void link.set(dst, concat(link.get(null) || [], [src])),
       void link.delete(null),
       Left(link))
     : Right(link))

 .reduce<Map<T | null, T[]>>((link, src) =>
   dsts.length === 0
     ? link.set(null, concat(link.get(null) || [], [src]))
     : dsts
         .reduce<Either<typeof link, typeof link>>((m, dst) =>
           m.bind(link =>
             !link.has(dst) && compare(src, dst)
               ? (
                 void link.set(dst, concat(link.get(null) || [], [src])),
                 void link.delete(null),
                 Left(link))
               : Right(link))
         , Right(link))
         .fmap(link =>
           link.set(null, concat(link.get(null) || [], [src])))
         .extract(link =>
           link)

 .fmap(a =>
   concat(as, [a])))

 p => Right(tuple([sp, concat(ap, [p])])))))

 m2.fmap(s2 =>
   concat(s1, s2))))))