  The anomalous behavior of 2-dimensional factorial moment in nucleus-nucleus
collisions is studied in some detail using both mini-bias and central collision
data of ^{16}O-, ^{32}S- and ^{197}Au-Em interactions from EMU01 experiment.
The correct value for the effective Hurst exponent in the analysis of
higher-dimensional factorial moment is found to be greater than unity, showing
clearly the existence of superposition effect in nucleus-nucleus collisions.
