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Proof step 4 from proof last-in (previous step)

⊦ str = "", s→l(str).last ∈ str

User applied rule structural induction

And obtained the following new goals

Proof step 5:
str0 = "" ∨ s→l(str0).last ∈ str0 ⊦ char + str0 = "", s→l(char + str0).last ∈ char + str0

Rule argument

indvar: str

Additional information

Used simplifier rules: string freely generated by "", +;;