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Description: A suffix of a concatenation is either a suffix of the second concatenated word or a concatenation of a suffix of the first word with the second word. (Contributed by Alexander van der Vekens, 31-Mar-2018) (Revised by Alexander van der Vekens, 30-May-2018) (Proof shortened by AV, 14-Oct-2022)
| Ref | Expression | ||
|---|---|---|---|
| Hypothesis | swrdccatin2.l | ⊢ 𝐿 = ( ♯ ‘ 𝐴 ) | |
| Assertion | swrdccat3b | ⊢ ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) → ( 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) → ( ( 𝐴 ++ 𝐵 ) substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) = if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) ) ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | swrdccatin2.l | ⊢ 𝐿 = ( ♯ ‘ 𝐴 ) | |
| 2 | simpl | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ) | |
| 3 | simpr | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) | |
| 4 | elfzubelfz | ⊢ ( 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) → ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) | |
| 5 | 4 | adantl | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) |
| 6 | 1 | pfxccat3 | ⊢ ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) → ( ( 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ∧ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → ( ( 𝐴 ++ 𝐵 ) substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) = if ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 , ( 𝐴 substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) , if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ ( 𝐵 prefix ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) ) ) ) ) ) |
| 7 | 6 | imp | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ ( 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ∧ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ) → ( ( 𝐴 ++ 𝐵 ) substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) = if ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 , ( 𝐴 substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) , if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ ( 𝐵 prefix ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) ) ) ) ) |
| 8 | 2 3 5 7 | syl12anc | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → ( ( 𝐴 ++ 𝐵 ) substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) = if ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 , ( 𝐴 substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) , if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ ( 𝐵 prefix ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) ) ) ) ) |
| 9 | 1 | swrdccat3blem | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ) → if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) = ( 𝐴 substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) ) |
| 10 | iftrue | ⊢ ( 𝐿 ≤ 𝑀 → if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) = ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) ) | |
| 11 | 10 | 3ad2ant3 | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ 𝐿 ≤ 𝑀 ) → if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) = ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) ) |
| 12 | lencl | ⊢ ( 𝐴 ∈ Word 𝑉 → ( ♯ ‘ 𝐴 ) ∈ ℕ0 ) | |
| 13 | 12 | nn0cnd | ⊢ ( 𝐴 ∈ Word 𝑉 → ( ♯ ‘ 𝐴 ) ∈ ℂ ) |
| 14 | lencl | ⊢ ( 𝐵 ∈ Word 𝑉 → ( ♯ ‘ 𝐵 ) ∈ ℕ0 ) | |
| 15 | 14 | nn0cnd | ⊢ ( 𝐵 ∈ Word 𝑉 → ( ♯ ‘ 𝐵 ) ∈ ℂ ) |
| 16 | 1 | eqcomi | ⊢ ( ♯ ‘ 𝐴 ) = 𝐿 |
| 17 | 16 | eleq1i | ⊢ ( ( ♯ ‘ 𝐴 ) ∈ ℂ ↔ 𝐿 ∈ ℂ ) |
| 18 | pncan2 | ⊢ ( ( 𝐿 ∈ ℂ ∧ ( ♯ ‘ 𝐵 ) ∈ ℂ ) → ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) = ( ♯ ‘ 𝐵 ) ) | |
| 19 | 17 18 | sylanb | ⊢ ( ( ( ♯ ‘ 𝐴 ) ∈ ℂ ∧ ( ♯ ‘ 𝐵 ) ∈ ℂ ) → ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) = ( ♯ ‘ 𝐵 ) ) |
| 20 | 13 15 19 | syl2an | ⊢ ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) → ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) = ( ♯ ‘ 𝐵 ) ) |
| 21 | 20 | eqcomd | ⊢ ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) → ( ♯ ‘ 𝐵 ) = ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) |
| 22 | 21 | adantr | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → ( ♯ ‘ 𝐵 ) = ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) |
| 23 | 22 | 3ad2ant1 | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ 𝐿 ≤ 𝑀 ) → ( ♯ ‘ 𝐵 ) = ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) |
| 24 | 23 | opeq2d | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ 𝐿 ≤ 𝑀 ) → 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 = 〈 ( 𝑀 − 𝐿 ) , ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) 〉 ) |
| 25 | 24 | oveq2d | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ 𝐿 ≤ 𝑀 ) → ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) = ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) 〉 ) ) |
| 26 | 11 25 | eqtrd | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ 𝐿 ≤ 𝑀 ) → if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) = ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) 〉 ) ) |
| 27 | iffalse | ⊢ ( ¬ 𝐿 ≤ 𝑀 → if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) = ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) | |
| 28 | 27 | 3ad2ant3 | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ ¬ 𝐿 ≤ 𝑀 ) → if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) = ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) |
| 29 | 20 | adantr | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) = ( ♯ ‘ 𝐵 ) ) |
| 30 | 29 | 3ad2ant1 | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ ¬ 𝐿 ≤ 𝑀 ) → ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) = ( ♯ ‘ 𝐵 ) ) |
| 31 | 30 | oveq2d | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ ¬ 𝐿 ≤ 𝑀 ) → ( 𝐵 prefix ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) = ( 𝐵 prefix ( ♯ ‘ 𝐵 ) ) ) |
| 32 | pfxid | ⊢ ( 𝐵 ∈ Word 𝑉 → ( 𝐵 prefix ( ♯ ‘ 𝐵 ) ) = 𝐵 ) | |
| 33 | 32 | adantl | ⊢ ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) → ( 𝐵 prefix ( ♯ ‘ 𝐵 ) ) = 𝐵 ) |
| 34 | 33 | adantr | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → ( 𝐵 prefix ( ♯ ‘ 𝐵 ) ) = 𝐵 ) |
| 35 | 34 | 3ad2ant1 | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ ¬ 𝐿 ≤ 𝑀 ) → ( 𝐵 prefix ( ♯ ‘ 𝐵 ) ) = 𝐵 ) |
| 36 | 31 35 | eqtr2d | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ ¬ 𝐿 ≤ 𝑀 ) → 𝐵 = ( 𝐵 prefix ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) ) |
| 37 | 36 | oveq2d | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ ¬ 𝐿 ≤ 𝑀 ) → ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) = ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ ( 𝐵 prefix ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) ) ) |
| 38 | 28 37 | eqtrd | ⊢ ( ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) ∧ ¬ ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 ∧ ¬ 𝐿 ≤ 𝑀 ) → if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) = ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ ( 𝐵 prefix ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) ) ) |
| 39 | 9 26 38 | 2if2 | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) = if ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ≤ 𝐿 , ( 𝐴 substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) , if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ ( 𝐵 prefix ( ( 𝐿 + ( ♯ ‘ 𝐵 ) ) − 𝐿 ) ) ) ) ) ) |
| 40 | 8 39 | eqtr4d | ⊢ ( ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) ∧ 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) ) → ( ( 𝐴 ++ 𝐵 ) substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) = if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) ) |
| 41 | 40 | ex | ⊢ ( ( 𝐴 ∈ Word 𝑉 ∧ 𝐵 ∈ Word 𝑉 ) → ( 𝑀 ∈ ( 0 ... ( 𝐿 + ( ♯ ‘ 𝐵 ) ) ) → ( ( 𝐴 ++ 𝐵 ) substr 〈 𝑀 , ( 𝐿 + ( ♯ ‘ 𝐵 ) ) 〉 ) = if ( 𝐿 ≤ 𝑀 , ( 𝐵 substr 〈 ( 𝑀 − 𝐿 ) , ( ♯ ‘ 𝐵 ) 〉 ) , ( ( 𝐴 substr 〈 𝑀 , 𝐿 〉 ) ++ 𝐵 ) ) ) ) |