Quick followup, IRS publication 550 says it all and fairly clearly:
https://www.irs.gov/publications/p550
Under the 'Nondividend Distributions' section:
Basis adjustment.
A nondividend distribution reduces the basis of your stock. It is not taxed until your basis in the stock is fully recovered. This nontaxable portion also is called a return of capital; it is a return of your investment in the stock of the company. If you buy stock in a corporation in different lots at different times, and you cannot definitely identify the shares subject to the nondividend distribution, reduce the basis of your earliest purchases first.
When the basis of your stock has been reduced to zero, report any additional nondividend distribution you receive as a capital gain. Whether you report it as a long-term or short-term capital gain depends on how long you have held the stock. See Holding Period in chapter 4.
