Libeskind's original proposal, which is titled Memory Foundations, underwent extensive revisions during collaboration with Larry Silverstein, and Skidmore, Owings & Merrill, whom Silverstein hired. Though Libeskind designed the site, the individual buildings have been designed by different architects. While not all of Liebeskind's ideas were incorporated into the final design, his design and the public support it garnered did solidify the principle that the original footprints of the Twin Towers should be turned into a memorial and not be used for commercial purposes. As a result, Liebeskind's lawyers at the New York firm of Wachtell Lipton embarked on the multi-year negotiation process to frame a master plan for the rebuilding. The first step in this process, completed in 2003, was the "swap" in which Silverstein gave up his rights to the footprints of the Twin Towers so that they could become a memorial, and in exchange received the right to build five new office towers around the memorial. The "swap" and the ensuing negotiations, which lasted for many years, have been referred to as the most complex real estate transaction in human history because of the complexity of the issues involved, the many stakeholders, and the difficulty of reaching consensus.