Some sequences prove to be substitutable for a given morpheme class in particular environments and not in others. This brings up a new relation of non-repeated substitutability, which can be indicated in these equations by a modification of the class symbols. For example, if we indicate morphemes like 'boy', 'king', by 'N' and morphemes like '– hood', '-dom', by 'Nn', we may write 'N Nn = N' ('boyhood', 'kingdom' replace 'life' in 'His — was obsessed with many fears'. Since the equation means that 'N Nn' is replaceable everywhere by 'N', and 'N' by 'N Nn', we might be led to think that we could also replace 'N' by 'N Nn' in the equation itself and not obtain 'N Nn Nn = N'. However, this is not in general the case, since 'boyhood' does not occur before 'hood'. In contrast, 'AN = N' is repeatable, so that we can derive 'AAN = N', and so on: 'old man', or 'old', 'lonely man' are both substitutable for 'man'.
- Harris (1951), a pag.265