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            Return an int64_t pointer
            if this is a signed integer, else return nullptr
          
std::int64_t* if_int64();
            If this->kind() == kind::int64,
            returns a pointer to the underlying integer. Otherwise, returns nullptr.
          
The return value is used in both a boolean context and to assign a variable:
if( auto p = jv.if_int64() ) return *p;
Constant.
No-throw guarantee.