$ l-lang-parser-rs export run.l
Definition my_evenb: tm :=
  (rec (abs "my_evenb"
    (abs "n"
      (mat (var "n") [
        ("S", ["x"],
          (mat (var "x") [
            ("S", ["x"],
              (app (var "my_evenb") (var "x")));
            ("O", [],
              (const "false"))
          ]));
        ("O", [],
          (const "true"))
      ])
    )
  )).

Definition main: tm :=
  (app my_evenb (app (const "S") (app (const "S") (app (const "S") (app (const "S") (const "O")))))).

$ l-lang-parser-rs export run.l
     = Some (const "true")
     : option tm