Count set bits in a bigint and a BitArray

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open System.Linq

let bitcount (b:bigint) =
    let mutable b = b
    let mutable count = 0
    while b <> bigint 0 do
        b <- b &&& (b-(bigint 1))
        count <- count + 1
    count

let bitcount2 (b:System.Collections.BitArray) = 
    (b:> System.Collections.IEnumerable).Cast<bool>()
    |> Seq.map (fun v -> if v then 1 else 0) 
    |> Seq.sum

let bitcount3 (b:System.Collections.BitArray) = 
    let mutable count = 0
    for bit in b do
        if bit then
            count <- count+1
    count

let bitcount4 (b:System.Collections.BitArray) = 
    let mutable count = 0
    for bit=0 to b.Length-1 do
        if b.Get(bit) then
            count <- count+1
    count

namespace System

namespace System.Linq

val bitcount : b:bigint -> int

Full name: Script.bitcount

val b : bigint

type bigint = System.Numerics.BigInteger

Full name: Microsoft.FSharp.Core.bigint

val mutable b : bigint

val mutable count : int

val bitcount2 : b:System.Collections.BitArray -> int

Full name: Script.bitcount2

val b : System.Collections.BitArray

namespace System.Collections

Multiple items
type BitArray =
  new : length:int -> BitArray + 5 overloads
  member And : value:BitArray -> BitArray
  member Clone : unit -> obj
  member CopyTo : array:Array * index:int -> unit
  member Count : int
  member Get : index:int -> bool
  member GetEnumerator : unit -> IEnumerator
  member IsReadOnly : bool
  member IsSynchronized : bool
  member Item : int -> bool with get, set
  ...

Full name: System.Collections.BitArray

--------------------
System.Collections.BitArray(length: int) : unit
System.Collections.BitArray(bytes: byte []) : unit
System.Collections.BitArray(values: bool []) : unit
System.Collections.BitArray(values: int []) : unit
System.Collections.BitArray(bits: System.Collections.BitArray) : unit
System.Collections.BitArray(length: int, defaultValue: bool) : unit

type IEnumerable =
  member GetEnumerator : unit -> IEnumerator

Full name: System.Collections.IEnumerable

type bool = System.Boolean

Full name: Microsoft.FSharp.Core.bool

module Seq

from Microsoft.FSharp.Collections

val map : mapping:('T -> 'U) -> source:seq<'T> -> seq<'U>

Full name: Microsoft.FSharp.Collections.Seq.map

val v : bool

val sum : source:seq<'T> -> 'T (requires member ( + ) and member get_Zero)

Full name: Microsoft.FSharp.Collections.Seq.sum

val bitcount3 : b:System.Collections.BitArray -> int

Full name: Script.bitcount3

val bit : bool

val bitcount4 : b:System.Collections.BitArray -> int

Full name: Script.bitcount4

val bit : int

property System.Collections.BitArray.Length: int

System.Collections.BitArray.Get(index: int) : bool