1. A method, executed by a computer system, for altering a base expression associated with an ontology, the method comprising: generating a canonical expression from the base expression; modifying the canonical expression to generate a modified canonical expression, the modification being the addition, replacement or deletion of a target update expression segment; modifying the base expression to generate a modified base expression by adding a modification sub-expression to the base expression or deleting a modification sub-expression from the base expression, the modification sub-expression being determined at least in part from the target update expression segment; generating a comparison canonical expression from the modified base expression; and comparing the comparison canonical expression with the modified canonical expression to determine whether to substitute the base expression with the modified base expression.