Title: Foreword: a Commemorative Issue for Alan L. Selman
Abstract: Theory of Computing Systems.To commemorate his extraordinary contributions to theoretical computer science, we solicited papers from the theory community.Nine groups responded to the call, which we present in this issue.-In "Holographic algorithms on domains of general size," a Zhiguo Fu and Jin-Yi Cai study the Simultaneously Realizability Problem in holographic algorithm construction.The authors construct a polynomial-time algorithm for the problem on domain size k ≥ 3 where the signatures are realizable by "matchgates" over a basis of size 1.-In "Subclasses of Ptime interpreted by programming languages," Siddharth Bhaskar, Cynthia Kop, and Jakob Grue Simonsen study the cons-free programming language of Neil Jones (who collaborated with Alan and, sadly, left this universe in April this year).They study the relationship between polynomial-step cons-free programs, polynomial-size circuits, and polynomial time-bounded Turing machines.They show that for each polynomial p, a polynomial-time decidable relation R exists that forces cons-free programs to spend polynomial time almost everywhere.-The paper "Dimension and the structure of complexity classes," by Jack Lutz, Neil Lutz, and Elvira Mayordomo investigates the Point-to-Set Principle in complexitytheoretic dimension theory, which connects the Hausdorff and the algorithm