Recursively find all files from a sequence of directories

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open System.IO
let allFilesInFolderList pattern folders = 
    folders |> Seq.collect (fun folder -> Directory.EnumerateFiles(folder, pattern, SearchOption.AllDirectories))

namespace System

namespace System.IO

val allFilesInFolderList : pattern:string -> folders:seq<string> -> seq<string>

val pattern : string

val folders : seq<string>

module Seq

from Microsoft.FSharp.Collections

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

val folder : string

type Directory =
  static member CreateDirectory : path: string -> DirectoryInfo
  static member Delete : path: string -> unit + 1 overload
  static member EnumerateDirectories : path: string -> IEnumerable<string> + 3 overloads
  static member EnumerateFileSystemEntries : path: string -> IEnumerable<string> + 3 overloads
  static member EnumerateFiles : path: string -> IEnumerable<string> + 3 overloads
  static member Exists : path: string -> bool
  static member GetCreationTime : path: string -> DateTime
  static member GetCreationTimeUtc : path: string -> DateTime
  static member GetCurrentDirectory : unit -> string
  static member GetDirectories : path: string -> string [] + 3 overloads
  ...

Directory.EnumerateFiles(path: string) : System.Collections.Generic.IEnumerable<string>
Directory.EnumerateFiles(path: string, searchPattern: string) : System.Collections.Generic.IEnumerable<string>
Directory.EnumerateFiles(path: string, searchPattern: string, enumerationOptions: EnumerationOptions) : System.Collections.Generic.IEnumerable<string>
Directory.EnumerateFiles(path: string, searchPattern: string, searchOption: SearchOption) : System.Collections.Generic.IEnumerable<string>

type SearchOption =
  | TopDirectoryOnly = 0
  | AllDirectories = 1

field SearchOption.AllDirectories: SearchOption = 1