Talk:Unrestricted grammar

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I believe the following is incorrect. Can someone confirm?

$P$ is a set of production rules of the form $\alpha \to \beta$ where $α$ and $β$ are in $N \cup \Sigma$

I say this because in this snippet:

If $β$ appears at some position on the second tape, replace $β$ by $γ$ at that point, possibly shifting the symbols on the tape left or right depending on the relative lengths of $β$ and $γ$ (e.g. if $β$ is longer than $γ$ , shift the tape symbols left).

... it looks like $α$ and $β$ are sequences, so not in $N \cup \Sigma$ , but in the set of all finite sequences using letters from the alphabet $N \cup \Sigma$ (I'm not sure the best way to notate this, maybe something from Sequence_space)

Pchiusano 02:49, 26 July 2006 (UTC)

Yes, you're right; someone must have written without really thinking. Ruakh 13:10, 26 July 2006 (UTC)