  Rules for in-medium complex particle production in nuclear reactions are
proposed. These rules have been implemented in two models to simulate
nucleon-nucleus and nucleus-nucleus reactions around the Fermi energy. Our work
emphasizes the effect of randomness in cluster formation, the importance of the
nucleonic Fermi motion as well as the role of conservation laws. The concepts
of total available phase-space and explored phase-space under constraint
imposed by the reaction are clarified. The compatibility of experimental
observations with a random clusterization is illustrated in a schematic
scenario of a proton-nucleus collision. The role of randomness under constraint
is also illustrated in the nucleus-nucleus case.
