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Proof step 1 from proof len-app (conclusion)

⊦ #(str + str0) = # str + # str0

User applied rule structural induction

And obtained the following new goals

Proof step 2:
∀ str0. #(str1 + str0) = # str1 + # str0 ⊦ #((char + str1) + str0) = #(char + str1) + # str0

Rule argument

indvar: str

Additional information

Used simplifier rules: Length-empty: ⊦ # "" = 0;%string freely generated by "", +;;%append-base: ⊦ "" + str = str; from specification string-append%assoc: ⊦ (str1 + str2) + str3 = str1 + str2 + str3; from specification string-append%add-base: ⊦ n + 0 = n; from specification nat-basic2%a: ⊦ (m + n0) + n1 = m + n0 + n1; from specification nat-basic2%c: ⊦ m + n = n + m; from specification nat-basic2