 This is often described as adding "twigs".By proving that every "n"-omino is a sequence of twigs, and by proving limits on the combinations of possible twigs, one obtains an upper bound on the number of "n"-ominoes.For example, in the algorithm outlined above, at each step we must choose a larger number, and at most three new numbers are added (since at most three unnumbered squares are adjacent to any numbered square).