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## Function left

boost::proto::left — Return the left child of the specified binary Proto expression.

## Synopsis

    // In header: <boost/proto/traits.hpp>

    template<typename Expr>
      typename proto::result_of::left< Expr & >::type left(Expr & expr);
    template<typename Expr>
      typename proto::result_of::left< Expr const & >::type
      left(Expr const & expr);

## Description

Return the left child of the specified binary Proto expression. The child is returned by reference.

Requires: | `proto::is_expr<Expr>::value` is `true`. `2 == Expr::proto_arity::value`
Returns: | A reference to the left child of `expr`.
Throws: | Will not throw.