The verbs listed in the previous section can be classified as auxiliaries based upon two diagnostics: they allow subject–auxiliary inversion (the type of inversion used to form questions etc.) and (equivalently) they can take not as a postdependent (a dependent that follows its head). The following examples illustrate the extent to which subject–auxiliary inversion can occur with an auxiliary verb but not with a full verb:[11] a. He was working today. b. Was he working today? - Auxiliary verb was allows subject–auxiliary inversion. a. He worked today. b. *Worked he today? - Full verb worked does not allow subject–auxiliary inversion. a. She can see it. b. Can she see it? - Auxiliary verb can allows subject–auxiliary inversion. a. She sees it. b. *Sees she it? - Full verb sees does not allow subject–auxiliary inversion. (The asterisk * is the means commonly used in linguistics to indicate that the example is grammatically unacceptable.) The following examples illustrate that the negation not can appear as a postdependent of a finite auxiliary verb, but not as a postdependent of a finite full verb:[12] a. Sam would try that. b. Sam would not try that. - The negation not appears as a postdependent of the finite auxiliary would. a. Sam tried that. b. *Sam tried not that. - The negation not cannot appear as a postdependent of the finite full verb tried. a. Tom could help. b. Tom could not help. - The negation not appears as a postdependent of the finite auxiliary could. a. Tom helped. b. *Tom helped not. - The negation not cannot appear as a postdependent of the finite full verb helped. A third diagnostic that can be used for identifying auxiliary verbs is verb phrase ellipsis. Auxiliary verbs can introduce verb phrase ellipsis, but main verbs cannot.[citation needed] See the article on verb phrase ellipsis for examples. Note that these criteria lead to the copula be being considered an auxiliary (it undergoes inversion and takes postdependent not, e.g. Is she the boss?, She is not the boss). However, if one defines auxiliary verb as a verb that somehow "helps" another verb, then the copula be is not an auxiliary, because it appears without another verb. The literature on auxiliary verbs is somewhat inconsistent in this area In the types of VP-ellipsis considered here, which are features of English grammar, the elided VP must be a non-finite VP; it cannot be a finite VP. Further, the ellipsis must be introduced by an auxiliary verb (be, can, do, don't, could, have, may, might, shall, should, will, won't, would, etc.) or by the infinitive particle to.[3] In the examples below, the elided material of VP-ellipsis is indicated using subscripts and a smaller font and the antecedent to the ellipsis is bolded: You might do it, but I won't do it. She won't laugh, but he will laugh. Susan has been cheating, and Fred has been cheating too. Larry is not telling the truth, neither is Jim telling the truth. Attempts at VP-ellipsis that lack an auxiliary verb fail, unless the infinitive particle to is retained: a. *Sam wants to eat, and Fred wants to eat as well. (* indicates that the sentence is ungrammatical) b. Sam wants to eat, and Fred wants to eat as well. a. *Josh likes to sleep late, and Hillary likes to sleep late as well. b. Josh likes to sleep late, and Hillary likes to sleep late as well. Apparent exceptions to this restriction on VP-ellipsis may be instances of null complement anaphora, e.g. ?Bill tried to leave, and Jim tried to leave too. A particularly frequent construction in which VP-ellipsis (obligatorily) occurs is the tag question: Jeremy likes beer, doesn't he like beer? - Tag question involves VP-ellipsis Susan will write the paper, won't she write the paper. - Tag question involves VP-ellipsis