We implemented the model in stochastic simulations which allows tracking of all individual fibers and their compositions. Simulations started from replicator fibers. In this work we did not consider the nucleation of new fibers, only the growth of existing ones. For simplicity also fiber breakage was ignored. Fibers were assumed to grow from one end, following recent experimental observations by high-speed atomic force microscopy . Supporting Information (SI) section S1 gives an example of a simulation that was followed until all trimers were converted to hexamers.