[/==============================================================================
    Copyright (C) 2001-2010 Joel de Guzman
    Copyright (C) 2001-2005 Dan Marsden
    Copyright (C) 2001-2010 Thomas Heller

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]

[section Actor]

The `Actor` is the main concept behind the library. Actors are function objects.
An actor can accept 0 to `BOOST_PHOENIX_LIMIT` arguments.

[note You can set `BOOST_PHOENIX_LIMIT`, the predefined maximum arity an
actor can take. By default, `BOOST_PHOENIX_LIMIT` is set to 10.]

Phoenix supplies an `actor` class template whose specializations
model the `Actor` concept.  `actor` has one template parameter, `Expr`,
that supplies the underlying expression to evaluate.

    template <typename Expr>
    struct actor
    {
        return_type
        operator()() const;
        
        return_type
        operator()();

        template <typename T0>
        return_type
        operator()(T0& _0) const;

        template <typename T0>
        return_type
        operator()(T0 const& _0) const;

        template <typename T0>
        return_type
        operator()(T0& _0);

        template <typename T0>
        return_type
        operator()(T0 const& _0);


        //...
    };

The actor class accepts the arguments through a set of function call operators
for 0 to `BOOST_PHOENIX_LIMIT` arities (Don't worry about the details, for now. Note, for example,
that we skimed over the details regarding `return_type`). The arguments
are then forwarded to the actor's `Expr` for evaluation.

[endsect]