Explain Seq.unfold verbosely

(In learning from "Programming F# 3.0, 2nd Edition")

Seq.unfold is a function hard-to-understand if I read any Microsoft's documents and any books. For example, the way in "Programming F# 3.0, 2nd Edition" is hard to understand.

> // Generate the next element of the Fibonacci sequence given the previous
// two elements. To be used with Seq.unfold.
let nextFibUnder100 (a, b) =
    if a + b > 100 then
        None
    else
        let nextValue = a + b
        Some(nextValue, (nextValue, a));;

val nextFibUnder100 : int * int -> (int * (int * int)) option

> let fibsUnder100 = Seq.unfold nextFibUnder100 (0, 1);;

val fibsUnder100 : seq<int>

> Seq.toList fibsUnder100;;
val it : int list = [1; 1; 2; 3; 5; 8; 13; 21; 34; 55; 89]

But, since I re-write this example verbosely like below, I can understand about it.

module UnfoldVerbosely =
    let fibSeed (current, next) =
        let yieldValue = current
        let next'current = next
        let next'next = current + next
        Some(yieldValue, (next'current, next'next))
    let fib = Seq.unfold fibSeed (1L, 1L)

printfn "%A" (Seq.take 2 UnfoldVerbosely.fib)
printfn "%A" (Seq.take 3 UnfoldVerbosely.fib)
printfn "%A" (Seq.take 4 UnfoldVerbosely.fib)
printfn "%A" (Seq.take 5 UnfoldVerbosely.fib)

module UnfoldVerbosely = begin
  val fibSeed : current:int * next:int -> (int * (int * int)) option
  val fib : seq<int>
end

seq [1; 1]
seq [1; 1; 2]
seq [1; 1; 2; 3]
seq [1; 1; 2; 3; ...]