{1, 2, 3}.frozen()

shows real originality, arising from deep creative thought about the roots of the problem. I was both surprised and delighted when I saw it, and I think some others were too. (I agree with others that here 'freeze' is better than 'frozen'.)

Obviously, each of the two proposals

   f{1, 2, 3}

   {1, 2, 3}.freeze()

has advantages and disadvantages when compared to the other.

Both deserve full and fair consideration.

-- 

Jonathan