type Expression =
  | Variable of string
  | Const of Value
  | Binary of Op * Expr * Expr
  | Func of string * Expr list

type Command =
  // Program commands
  | Print of Expr * bool
  | Goto of int
  | Poke of Expr * Expr
  | Assign of string * Expr
  | If of Expr * Command
  | Get of string

  // Editor and interactive
  | List of int option * int option
  | Delete of int option * int option
  | Run

/// Succeeds if the string 's'
/// starts with the prefix 'p'
let (|Prefix|_|) p s =
  if s.StartsWith p then
    Some(s.[p.Length..].Trim())
  else None

/// Succeeds if the string 's'
/// starts with a number; returns
/// the number & the remainder
let (|Number|_|) s =
  (...)

/// Parse command like '10 GOTO 20'
match input with
| Number(line, Prefix "GOTO" (Number(target, _))) ->

// Specify path(s) in a document
type Selector =
  | All
  | Index of string
  | Field of string
  | DotDot

// Two kinds of references
type RefKind =
  | Absolute
  | Relative

// Represents document structure
type Node =
  | Primitive of Primitive
  | Record of string * OrdList<string, Node>
  | List of string * OrdList<string, Node>
  | Reference of RefKind * Selector list

type Expr =
  // Lambda calculus
  | Application of Expr * Expr
  | Lambda of string * Expr
  | Variable of string
  // Numbers and functions
  | Constant of int
  | Binary of string * Expr * Expr
  | Unary of string * Expr
  // Data types
  | Tuple of Expr * Expr
  | TupleGet of bool * Expr
  | Case of bool * Expr
  | Match of Expr * string * Expr * Expr
  // Programming
  | Recursive of string * Expr * Expr
  | Let of string * Expr * Expr
  | If of Expr * Expr * Expr

// Example terms
// * 'human(socrates)' is a
//   predicate applied to atom
// * 'human(X)' is a predicate
//   applied to variable
type Term =
  | Atom of string
  | Variable of string
  | Predicate of string * Term list

// Clause with head and empty
// body represents a fact
type Clause =
  { Head : Term
    Body : Term list }