Communicating between in and out contracts

[Ary Borenszweig]

Kagamin wrote:
> Andrei Alexandrescu Wrote:
> 
>>     void push(T value);
>>     in {
>>        auto oldLength = length();
>>     }
>>     out {
>>        assert(value == top());
>>        assert(length == oldLength + 1);
>>     }
>>
>> Walter tried to implement that but it turned out to be very difficult
>> implementation-wise.
> 
> What is the problem? Syntactical, semantical or ABI-related?

This. What does it mean "implementation-wise"?