Well, what about this program?:

    -- factorial
    { {extern "manool.org.18/std/0.2/all"} in
    : let
      { Fact =
        { proc { N } as
        : unless N.IsI48[] & (N >= 1 - 1) signal
        {if (~)[N.IsI48[]] then TypeMismatch else InvariantViolation}
        else
        : var { Res = 1 } in
        : do Res after
        : while N <> 0 do Res = N * Res; N = N + ~1
        }
      }
      in
      Out.WriteLine[/* Out; */ "Factorial of 10 is "; Fact[10]$]
    }

It has absolutely all syntactic features and even makes sense!