1. A method for determining a variably sized context model from input signal data having symbols chosen from an alphabet having n symbols {a 1 , a 2 ,. .. , a n }, the method comprising: counting a number of occurrences of metasymbols in the input signal data formed by a context of K symbols and a currently considered symbol; preparing a context tree, each node having n children, the context tree having leaf nodes, each leaf node associated with a particular context derived from a path of edges and nodes leading to the leaf node from a root node of the context tree; and pruning the context tree by an ordered sequence of merge operations that each merges the children of a merge-candidate node.