\[ \newcommand{\tr}{\Rightarrow} \newcommand{\trs}{\tr^{\!\ast}} \newcommand{\rlnm}[1]{\mathsf{(#1)}} \newcommand{\rred}[1]{\xrightarrow{#1}} \newcommand{\rreds}[1]{\mathrel{\xrightarrow{#1}\!\!^*}} \newcommand{\cl}{\mathsf{Cl}} \newcommand{\pow}{\mathcal{P}} \newcommand{\matches}{\mathrel{\mathsf{matches}}} \newcommand{\kw}[1]{\mathsf{#1}} \newcommand{\andop}{\mathrel{\&\!\&}} \newcommand{\orop}{\parallel} \newcommand{\ff}{\mathsf{false}} \newcommand{\tt}{\mathsf{true}} \newcommand{\abra}[1]{\langle #1 \rangle} \newcommand{\bnfnt}[1]{\abra{\small \textsf{#1}}} \newcommand{\llbracket}{[\![} \newcommand{\rrbracket}{]\!]} \newcommand{\first}{\mathsf{First}} \newcommand{\nullable}{\mathsf{Nullable}} \newcommand{\follow}{\mathsf{Follow}} \newcommand{\tm}[1]{\mathsf{#1}} \newcommand{\nt}[1]{\mathit{#1}} \newcommand{\Coloneqq}{::=} \newcommand{\abs}[1]{|#1|} \] 
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(** A [form] is either a top-level definition or an expression to be evaluated .*)
type form =
  | Define of string * sexp
  | Expr of sexp

(** A [sexp] is an expression to be evaluated. *)
and sexp =
  | Bool of bool
  | Num of int
  | Ident of string
  | Lambda of string list * sexp
  | Call of primop * sexp list
  | App of sexp * sexp list

(** A [prog] is just a list of [form]. *)
type prog = form list