1. A method of comparing a first pattern represented by a first ordered set of elements each having a value with a second pattern represented by a second ordered set of element each having a value, comprising performing, for each of a plurality of elements of the first ordered set the steps of: (i) selecting from the first ordered set a plurality of elements in the vicinity of the element under consideration, the selected elements having, within the ordered set, respective mutually different positions relative to the element under consideration; (ii) selecting an element of the second ordered set; (iii) comparing the selected plurality of elements of the first ordered set with a like plurality of elements of the second ordered set each of which has within the second ordered set the same position relative to the selected element of the second ordered set as a respective one of the selected plurality of elements of the first ordered set has relative to the element under consideration, said comparison comprising comparing the value of each of the selected plurality of elements of the first ordered set with the value of the correspondingly positioned element of the like plurality of elements of the second ordered set in accordance with a predetermined match criterion to produce a decision that the plurality of elements of the first ordered set matches the plurality of elements of the second ordered set; (iv) repeating said comparison wherein at least some comparisons are made with a fresh selection of the plurality of elements of the first ordered set and at least some comparisons are made with a fresh selection of an element of the second ordered set; and (v) generating for the element under consideration a similarity measure as a function of the number of comparisons for which the comparison indicates a match.