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Proof step 1 from proof substring-zero-rec (conclusion)

⊦ n ≠ 0 ∧ str ≠ "" → substring(str, 0, n) = str.char1 + substring(str.rstring, 0, n -1)

Heuristic simplifier applied rule simplifier

And obtained the following new goals

Proof step 2:
⊦ str = "", n = 0, substring(str, 0, n) = str.char1 ' + substring(str.rstring, 0, n - 1)

Rule argument

Additional information

Used simplifier rules: chartostring-01: ⊦ char + str = char ' + str; from specification string-append%pred-03: ⊦ n -1 = n - 1; from specification nat