jack

I'm not completely sure, I don't exactly think that, so you're probably better off asking SimonT. But what I *think* it means is that, if you're thinking in terms of boolean logic, AND, OR, NOR, XOR, etc, if "I'm happy but a bit nervous" is true, then "I'm happy" and "I'm a bit nervous" are both true, just as if you'd said "I'm happy and a bit nervous". (But that ignores the difference between "and" and "but" that "but" suggests you're a bit less happy than you might be otherwise.)