1. A computer tool comprising: a working memory to receive input data, an auxiliary memory to store several approximation dictionaries, a reducer arranged to calculate iteratively approximation data on the input data, in which the successive iterations are of the matching pursuit type, a selector arranged to select an approximation dictionary from the several approximation dictionaries in the auxiliary memory as a function of a selection rule based on at least some of the preceding iterations, in which: the reducer calls the selector in each iteration, and uses the dictionary thus selected in the matching pursuit; the selection rule comprises the selection of the approximation dictionary as a function of the number of the current iteration and/or the approximation data calculated in the preceding iterations; the approximation dictionaries are interconnected by a tree structure, and the selection rule comprises the selection of the approximation dictionary at a level of the tree taken from the number of the current iteration.