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By convention for the case where <math>k = 0,\!</math> this gives <math>\underline{\underline{X}}^0 = \{ () \},</math> that is, the singleton set consisting of the empty sequence.  Depending on the setting, the empty sequence is referred to as the ''empty word'' or the ''empty sentence'', and is commonly denoted by an epsilon <math>{}^{\backprime\backprime} \varepsilon {}^{\prime\prime}</math> or a lambda <math>{}^{\backprime\backprime} \lambda {}^{\prime\prime}.</math>  In this text, I use the symbol "!" (a stricken exclamation point) as a synonym for the empty sequence <math>().\!</math>  In addition, I use the symbol "!" (a stricken exclamation point underscored) as a synonym for <math>\{ () \}.\!</math>

By convention for the case where <math>k = 0,\!</math> this gives <math>\underline{\underline{X}}^0 = \{ () \},</math> that is, the singleton set consisting of the empty sequence.  Depending on the setting, the empty sequence is referred to as the ''empty word'' or the ''empty sentence'', and is commonly denoted by an epsilon <math>{}^{\backprime\backprime} \varepsilon {}^{\prime\prime}</math> or a lambda <math>{}^{\backprime\backprime} \lambda {}^{\prime\prime}.</math>  In this text a variant epsilon symbol will be used for the empty sequence, <math>\varepsilon = ().\!</math>  In addition, a singly underlined epsilon will be used for the language that consists of a single empty sequence, <math>\underline\varepsilon = \{ \varepsilon \} = \{ () \}.\!</math>

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