  We present a computation model based on a subclass of GP 2 graph programs
which can simulate any off-line Turing machine of space complexity O(s(n) log
s(n)) in space O(s(n)). The simulation only requires a quadratic time overhead.
Our model shares this property with Sch\"onhage's storage modification machines
and Kolmogorov-Uspenskii machines. These machines use low-level pointer
instructions whereas our GP 2-based model uses pattern-based transformation
rules and high-level control constructs.
