In many cases substitution occurs only in the environment of some particular class or sequence. E.g. one member of 'A' is replaceable by two, but only if a member of 'N' follows: 'fine' is replaceable by 'fine young' in 'They are fine men', but not in 'They are fine'. Instead of saying that 'AA = A' but only before 'N', we avoid the extra comment outside the equation by writing 'AAN = AN' (or more simply 'AN = N', from which this can be derived). This equation provides only for the substitution which ocurs, and leaves no basis for replacing 'A' by 'AA' elsewhere.
- Harris (1951), a pag.265, n.11