anchor.diff:      | Surround of left : Command * mid : Command * right : Command
       | Indent of Command
       | VSkip
  -    | VSep of cmds : Command list * separator : Command
  -    | HSep of cmds : Command list * separator : Command
  +    | VSep of cmds : Command seq * separator : Command
  +    | HSep of cmds : Command seq * separator : Command
       | Nop
   
   let fmt fstr xs = System.String.Format(fstr, Seq.toArray xs) |> String
  
positive.code: 
  let equality = binop "="
  
  let ivsep = vsep >> Indent
  
  let headed name cmds = 
      vsep cmds
      |> Indent
      |> (curry Header) name
  
  let keyMap = 
      List.map (fun (k, v) -> 
  
negative.code: open Starling.Utils
  
  /// Type of pretty-printer commands.
  [<NoComparison>]
  type Command = 
      | Header of heading : string * Command
      | Separator
  