4. A method of claim 2 , further comprising the steps of: retrieving the base expression from a database storing ontology expressions before a canonical expression is generated from it; and storing the last modified base expression in the database when the comparison canonical expression matches the modified canonical expression.
2. A method of claim 1 , wherein the steps of modifying the base expression, generating a comparison canonical expression and comparing the comparison canonical expression with the modified canonical expression are sequentially repeated, the modification sub-expression including one or more elements and being modified to include a larger number of elements with each repetition, until the comparison canonical expression matches the modified canonical expression.