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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 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>

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>

It is probably worth remarking at this point that all empty sequences are indistinguishable (in a one-level formal language, that is), and thus all sets that consist of a single empty sequence are identical.  Consequently, <math>\underline{\underline{X}}^0 = \{ () \} = \underline{\varepsilon} = \underline{\underline{Y}}^0,</math> for all resources <math>\underline{\underline{X}}</math> and <math>\underline{\underline{Y}}.</math>  However, the empty language <math>\varnothing = \{ \}</math> and the language that consists of a single empty sequence <math>\underline\varepsilon = \{ \varepsilon \} = \{ () \}\!</math> need to be distinguished from each other.

It is probably worth remarking at this point that all empty sequences are indistinguishable (in a one-level formal language, that is), and thus all sets that consist of a single empty sequence are identical.  Consequently, <math>\underline{\underline{X}}^0 = \{ () \} = \underline{\varepsilon} = \underline{\underline{Y}}^0,</math> for all resources <math>\underline{\underline{X}}</math> and <math>\underline{\underline{Y}}.</math>  However, the empty language <math>\varnothing = \{ \}</math> and the language that consists of a single empty sequence <math>\underline\varepsilon = \{ \varepsilon \} = \{ () \}</math> need to be distinguished from each other.

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