timyates

fact_improver = { partial ->
  { BigInteger n ->
    n == 0 ? 1 : n * partial( n - 1 )
  }
}

y = { improver ->
  { gen -> { BigInteger n -> improver( gen( gen ) )( n ) } } { gen ->
           { BigInteger n -> improver( gen( gen ) )( n ) } }
}