identity substitution造句

The increasing operation in step 3 is sometimes called " shift " and written ?! " k " where " k " is a natural number indicating the amount to increase the variables; For example, ?! 0 is the identity substitution, leaving a term unchanged.
Starting with " G " being the unification problem to be solved and " S " being the identity substitution, rules are applied nondeterministically until the empty set appears as the actual " G ", in which case the actual " S " is a unifying substitution.
It's difficult to find identity substitution in a sentence. 用identity substitution造句挺难的