entropi wrote:
The diagram is the result of a joseki. It's whites turn now and he will probably tenuki.
My question: is there a reason for black not to play at a? It looks like definitely sente. Does it remove any aji, if yes which one?
I know the black stone above is sort of sacrificed for getting the corner and it's difficult to make something out of it. But it looks like it still has potential for creating some aji later, depending on the fight going on at the left side.
Therefore, in the exchange (Black a-White b), White-b looks more useless than Black-a. If so, (why) is the exchange bad for black?
Sorry if the question is silly but I would like to learn the idea behind avoiding such moves. Thanks for the answers in advance?
"c" is the only other logical peep aji, and may have situations where you'd want to play it. The fact it makes "d" into not only cutting the stones off below, but because of the atari doing so in sente, makes it a large peep. If the exchange of "a" and "b" has already happened, White can choose to sacrificing the two White stones quite easily - in which case both "c" and "d" would be gote for Black for a relatively small gain as there's no atari for White to connect after "d". Also "a" just makes Black a bit bigger and heavier. Sure, it may be sente, but it's also a big ko threat (though White I would have thought would opt to play "a" himself before that sort of ko appeared - at least, I would).
That would be my take anyway, take it with a grain of salt
are more valuable than saving the two stones.
is considered joseki for long time but now it is considered inferior for white. So if 
"Black 5 attempts to connect underneath, to no avail", which probably means that it is indeed not suggested as joseki. But why is it then suggested at all, and with the continuation which seems to be even more wrong 
)