A "morpheme" then is "the smallest phonologically distinguishable bound or unbound formal modification of a lexeme which is paradigmatically ordered in a closed class, marks grammatical functions", i.e. is associated only with internal grammatical referents in some context, "and is not susceptible to L-derivation".
- Beard (2004), Pag. 509

Morphemes are instructions for constituting phonological modifications of lexemes. They are not directly related to grammatical functions but are separated from them by paradigmatically organized M-rules. [...] "The lexeme presupposes a prespecified segmental sequence; the morpheme presupposes the lexeme".
- Beard (2004), Pag. 508