    "A reference to a function. The type arguments encode the
     [[return type|Return]] of the function along with its
     [[parameter types|Arguments]]. The parameter types are
     represented by a tuple type. Functions declared `void`
     are considered to have the return type `Anything`.

     For example, the type of the anonymous function
     `(Float x, Integer y) => x^y+1` is:

         Callable<Float, [Float,Integer]>

     which we usually abbreviate to `Float(Float,Integer)`.

     Likewise, the type of the function reference `plus<Float>`
     to the function [[plus]] is:

         Callable<Float, [Float,Float]>

     which we abbreviate as `Float(Float,Float)`.

     A variadic function is represented using an unterminated
     tuple type. For example, the type of the function reference
     `concatenate<Object>` to the function [[concatenate]] is:

         Callable<Object[], [{Object*}*]>

     which we usually abbreviate `Object({Object*}*)`.

     A function with defaulted parameters is represented using
     a union type. For example, the type of the method reference
     `process.writeLine` to the method [[process.writeLine]] is:

         Callable<Anything, [String]|[]>

     which we usually abbreviate `Anything(String=)`.

     Any instance of `Callable` may be _invoked_ by supplying a
     positional argument list:

         Float(Float,Float) add = plus<Float>;
         value four = add(2.0, 2.0);

     or by supplying a tuple containing the arguments:

         Float(Float,Float) add = plus<Float>;
         [Float,Float] twoAndTwo = [2.0, 2.0];
         value four = add(*twoAndTwo);

     The type of the tuple must be assignable to the type
     argument of `Arguments`.

     This interface may not be implemented by user code."
    see (`class Tuple`)
    tagged("Functions")
    shared native interface Callable<out Return, in Arguments>
            given Arguments satisfies Anything[] {}