Pioneering work on the theory of the stored-program computer as laid down by Alan Turing was done at the IAS by John von Neumann, and the IAS machine built in the basement of the Fuld Hall from 1942 to 1951 under von Neumann's direction introduced the basic architecture of all modern digital computers. The IAS is the leading center of research in string theory and its generalization M-theory introduced by Edward Witten at the IAS in 1995. The Langlands program, a far-reaching approach which unites parts of geometry, mathematical analysis, and number theory was introduced by Robert Langlands, the mathematician who now occupies Albert Einstein's old office at the institute. Langlands was inspired by the work of Hermann Weyl, André Weil, and Harish-Chandra, all scholars with wide-ranging ties to the Institute, and the IAS maintains the key repository for the papers of Langlands and the Langlands program. The IAS is a main center of research for homotopy type theory, a modern approach to the foundations of mathematics which is not based on classical set theory. A special year organized by Institute professor Vladimir Voevodsky and others resulted in a benchmark book in the subject which was published by the Institute in 2013.