> You can put the first disk on the second or the thirth peg.

Think a, b, c as 'src, target, temp'

So if you have only one disk on src peg, it has to go target peg. But if you have two or more discs, you cannot put first peg on target peg.

hanoi 1 1 2 3 = [ (1, 2) ]
hanoi 2 1 2 3 = [ ?