Additionally, U.S. copyright law states that a "system" is ineligible for copyright protection. Research conducted by Malamud and Sprigman found that the 10th edition of the "Bluebook", published in 1958, had fallen into the public domain because its copyright had not been renewed, as required by U.S. law at the time. On October 6, 2014, Sprigman sent a letter of response to the Harvard Law Review Association, disclosing these findings and arguing that the content of the then-current 19th edition was nearly identical to the 10th barring trivial changes. Thus, he also announced an intent to publish a free-content version of the "Bluebook" known as "Baby Blue", which would be adapted from the public domain text of the 10th edition with "newly-created material that implements the Bluebook's system of citation in a fully usable form." Sprigman explained that "every person, including every poor person, should be able to cite the law. Imprisoned litigants, pro se litigants, legal clinics, small law firms and solo practitioners — all of them need better access to our system of legal citation if the law is to work for them and for their clients. And that means free access." Sprigman also stated that the use of an open-source development model and licensing would allow others to contribute to and help improve the system; he argued that the "Bluebook" in its current form was "over-prescriptive and rigid" and "a barrier to entry to our legal system," going on to ask, "what other standard of this importance to the American public would be entrusted to a group so small, unrepresentative, closed to input, and beyond both supervision and discipline?" In December 2015, following Twitter postings by Malamud teasing the upcoming release of "Baby Blue", the Harvard Law Review Association threatened legal action against the project, as it believed that the name "Baby Blue" had a confusing similarity to the "Bluebook" trademark, and requested a copy of the publication to perform intellectual property examinations under a presumption that it may be substantially similar to the copyrighted work. Sprigman objected to the trademark claims, feeling that "the idea they own the name 'blue' for a manual for legal citations is ridiculous." Following the threats, a group of over 120 Yale Law School students issued a letter in support of the "Baby Blue" project. In response to the trademark concerns, the name of the guide was changed to "The Indigo Book" on March 31, 2016.