Generalized ´write-once´ memories

Storage media such as digital optical discs, PROM\´s, or punched cards consist of a number of write-once bit positions (WIT\´s); each WIT initially contains a "0" that may later be irreversibly overwritten with a "r\´. Rivest and Shamir have shown that such write-once memories (WOM\´s) can be reused very efficiently. Generalized WOM\´s are considered, in which the basic storage element has more than two possible states and the legal state transitions are described by an arbitrary directed acyclic graph. The capabilities of such memories depend on the depth of the graphs rather than on their size, and the decision problem associated with the generalized WOM\´s in NP-hard even for -ary symbols rewritten several times or multiple values rewritten once.