why can't you trade specific techs?

[Silent Sorrow]

I've started fooling around with offering tech as gifts and for trade. But the reason I don't like it is there doesn't appear to be a way to give specific levels of tech--you have to give the entire area.

So, for example, you can't give explosive warheads II--you have to give the whole thing. This is even worse when you're talking about research areas--say, construction. Here you're obliged to give up an entire area of tech that allows other tech under it.

So I think the trading of specific techs, by level, should be put into the game if possible.