Error:
/src/main/main.pony:12:34: receiver type is not a subtype of target type
    var e_val = e_approx.evaluate()
                                 ^
    Info:
    /src/main/main.pony:12:17: receiver type: Eapproximator tag
        var e_val = e_approx.evaluate()
                    ^

    /src/main/main.pony:6:3: target type: Eapproximator box
      fun evaluate() : F64 =>
      ^
    /src/main/main.pony:3:3: Eapproximator tag is not a subtype of Eapproximator box: tag is not a subcap of box
      new create(step' : F64) =>
      ^