为什么我从编译器获得"expected unqualified-id before »}«"？

我从这里复制粘贴这段代码:

#include <boost/math/distributions/poisson.hpp>
namespace boost { namespace math {
template <class RealType = double, 
          class Policy   = policies::policy<> >
class poisson_distribution;
typedef poisson_distribution<> poisson;
template <class RealType, class Policy>
class poisson_distribution
{ 
public:
  typedef RealType value_type;
  typedef Policy   policy_type;
  poisson_distribution(RealType mean = 1); // Constructor.
  RealType mean()const; // Accessor.
}
}} // namespaces boost::math

并得到一个编译错误:

expected unqualified-id before »}«