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## Struct template fold

boost::proto::fold — A PrimitiveTransform that invokes the `fusion::fold<>` algorithm to accumulate a value.

## Synopsis

    template<typename Sequence, typename State0, typename Fun>
    struct fold :  proto::transform< fold<Sequence, State0, Fun> > {
      template<typename Expr, typename State, typename Data>
      struct impl :  proto::transform_impl< Expr, State, Data > {
        // types
        typedef when<_, Sequence>                                        X;            // For exposition only
        typedef when<_, State0>                                          Y;            // For exposition only
        typedef typename boost::result_of<X(Expr, State, Data)>::type    seq;          // A Fusion sequence, for exposition only
        typedef typename boost::result_of<Y(Expr, State, Data)>::type    state0;       // An initial state for the fold, for exposition only
        typedef unspecified                                              fun;          // fun(v)(e,s) == when<_,Fun>()(e,s,v)
        typedef typename fusion::result_of::fold<seq, state0, fun>::type result_type;

        // public member functions
        result_type operator()(typename impl::expr_param,
                               typename impl::state_param,
                               typename impl::data_param) const;
      };
    };

## Description

For the complete description of the behavior of the `proto::fold<>` transform, see the documentation for the nested `proto::fold::impl<>` class template.