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Re: [tlaplus] Could it be possible to check the variable starting alphabet

Just want to ask how this would be possible with sequence, if you can guide.

It depends on what you consider is important for your specification. Since you are interested in the first and last elements of a sequence of characters, you could let your variable take a sequence of elements, perhaps somewhat as below.

  Letter,    \* set representing all letters, instantiated for example by a set {a,b,c} of model values
  a, b, c    \* representing individual letters
ASSUME {a,b,c} \subseteq Letter

VARIABLE myletter

Init == myletter = << a,b,a >>

UpdateLetter ==  \* non-deterministically change some letter
  \E i \in 1 .. Len(myletter) : \E l \in Letter : myletter' = [myletter EXCEPT ![i] = l]

GoodLetters ==   \* some state predicate corresponding to desired state
  /\ myletter[1] = a
  /\ myletter[Len(myletter)] = c

Hope this helps,

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