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            Return a double pointer
            if this is a double, else return nullptr
          
double const* if_double() const;
            If this->kind() == kind::double_,
            returns a pointer to the underlying double. Otherwise, returns nullptr.
          
The return value is used in both a boolean context and to assign a variable:
if( auto p = jv.if_double() ) return *p;
Constant.
No-throw guarantee.