Prints Binary Trees in horizontal orientation connected by pipes

PipeTree.fs

(*
           Prints Trees Like
           -----------------
           
[Node]
|
|_(1)_[Leaf]
|
|_(1)_[Node]
      |
      |_(2)_[Node]
      |     |
      |     |_(3)_[Leaf]
      |     |
      |     |_(3)_[Node]
      |           |
      |           |_(4)_[Leaf]
      |
      |_(2)_[Node]
            |
            |_(3)_[Node]
            |     |
            |     |_(4)_[Leaf]
            |
            |_(3)_[Leaf]           

*)

type BTree =
    | Node of BTree * BTree  
    | Leaf  
    | Empty


let pipe_print (bt:BTree) : string =
    

    let str = string
    let concat (s:string list) = String.Concat s
    
    let ind  num s = 
        if   num <= 0 
        then ""
        else String.replicate num s   
        
    let rec construct t dep apnd : string  =

        let lvl = dep+1|>str
        let pd  = "|     "
        let ws  = "      "
        match t with
            
        | Node(l,r) when l = Empty -> 
                concat
                    [ "[Node]\n"; 
                      apnd;"|\n";
                      apnd;"|_(";lvl;")_"; construct r (dep+1) (concat[apnd;ws]);       ] 
        | Node(l,r) when r = Empty -> 
                concat
                    [ "[Node]\n";
                      apnd;"|\n";
                      apnd;"|_(";lvl;")_"; construct l (dep+1) (concat[apnd;pd])        ] 
        | Node(l,r)  -> 
                concat
                    [ "[Node]\n";
                      apnd;"|\n";
                      apnd;"|_(";lvl;")_"; construct l (dep+1) (concat[apnd;pd]);"\n"; 
                      apnd;"|\n";
                      apnd;"|_(";lvl;")_"; construct r (dep+1) (concat[apnd;ws]);       ] 
        | Leaf      -> "[Leaf]"
        | Empty     -> "[Empty]"
    "\n" + construct bt 0 ""