Separate string with numbers by thousands

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let separateByThousands (text: string) =
    let sb = System.Text.StringBuilder ()

    let rec insertSpaceEvery3Chars textPos =
        if textPos < 0 then
            ()
        else if textPos % 3 = 0 && textPos > 0 then
            sb.Append (" ") |> ignore
            sb.Append (text.[textPos]) |> ignore
            insertSpaceEvery3Chars (textPos - 1)
        else
            sb.Append (text.[textPos]) |> ignore
            insertSpaceEvery3Chars (textPos - 1)

    insertSpaceEvery3Chars (text.Length - 1) // Start from the end.
    sb.ToString ()

val separateByThousands : text:string -> string

val text : string

Multiple items
val string : value:'T -> string

--------------------
type string = System.String

val sb : System.Text.StringBuilder

namespace System

namespace System.Text

Multiple items
type StringBuilder =
  new : unit -> StringBuilder + 5 overloads
  member Append : value:string -> StringBuilder + 23 overloads
  member AppendFormat : format:string * arg0:obj -> StringBuilder + 7 overloads
  member AppendJoin : separator:string * [<ParamArray>] values:obj[] -> StringBuilder + 5 overloads
  member AppendLine : unit -> StringBuilder + 1 overload
  member Capacity : int with get, set
  member Chars : int -> char with get, set
  member Clear : unit -> StringBuilder
  member CopyTo : sourceIndex:int * destination:Span<char> * count:int -> unit + 1 overload
  member EnsureCapacity : capacity:int -> int
  ...
  nested type ChunkEnumerator

--------------------
System.Text.StringBuilder() : System.Text.StringBuilder
System.Text.StringBuilder(capacity: int) : System.Text.StringBuilder
System.Text.StringBuilder(value: string) : System.Text.StringBuilder
System.Text.StringBuilder(value: string, capacity: int) : System.Text.StringBuilder
System.Text.StringBuilder(capacity: int, maxCapacity: int) : System.Text.StringBuilder
System.Text.StringBuilder(value: string, startIndex: int, length: int, capacity: int) : System.Text.StringBuilder

val insertSpaceEvery3Chars : (int -> unit)

val textPos : int

System.Text.StringBuilder.Append(value: System.ReadOnlyMemory<char>) : System.Text.StringBuilder
   (+0 other overloads)
System.Text.StringBuilder.Append(value: System.ReadOnlySpan<char>) : System.Text.StringBuilder
   (+0 other overloads)
System.Text.StringBuilder.Append(value: char []) : System.Text.StringBuilder
   (+0 other overloads)
System.Text.StringBuilder.Append(value: obj) : System.Text.StringBuilder
   (+0 other overloads)
System.Text.StringBuilder.Append(value: uint64) : System.Text.StringBuilder
   (+0 other overloads)
System.Text.StringBuilder.Append(value: uint32) : System.Text.StringBuilder
   (+0 other overloads)
System.Text.StringBuilder.Append(value: uint16) : System.Text.StringBuilder
   (+0 other overloads)
System.Text.StringBuilder.Append(value: decimal) : System.Text.StringBuilder
   (+0 other overloads)
System.Text.StringBuilder.Append(value: float) : System.Text.StringBuilder
   (+0 other overloads)
System.Text.StringBuilder.Append(value: float32) : System.Text.StringBuilder
   (+0 other overloads)

val ignore : value:'T -> unit

property System.String.Length: int with get

System.Text.StringBuilder.ToString() : string
System.Text.StringBuilder.ToString(startIndex: int, length: int) : string