K is an executable semantic framework in which programming languages, calculi, as well as type systems or formal analysis tools can be defined making use of configurations, computations and rules. Configurations organize the system/program state in units called cells, which are labeled and can be nested. Computations carry "computational meaning" as special nested list structures sequentializing computational tasks, such as fragments of program; in particular, computations extend the original language or calculus syntax. K (rewrite) rules generalize conventional rewrite rules by making it explicit which parts of the term they read-only, write-only, or do not care about. This distinction makes K a suitable framework for defining truly concurrent languages or calculi even in the presence of sharing. Since computations can be handled like any other terms in a rewriting environment, that is, they can be matched, moved from one place to another in the original term, modified, or ...