In copyright-law (and I think patents?), there is the Threshold of originality. Basically you evaluate how much work and change went into something, to distinguish it from other works. Which IMHO comes down to how much of a and b can you still see in the mix, and how much did mixing them changed them in the end product. If the change is low, then people more likely consider it as not innovative.
Or how Homer Simpson once said: “People are afraid of new things. You should have taken an existing product and put a clock in it or something.”