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Mirrors > Home > ILE Home > Th. List > sbco | GIF version |
Description: A composition law for substitution. (Contributed by NM, 5-Aug-1993.) |
Ref | Expression |
---|---|
sbco | ⊢ ([𝑦 / 𝑥][𝑥 / 𝑦]𝜑 ↔ [𝑦 / 𝑥]𝜑) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | equsb2 1744 | . . 3 ⊢ [𝑦 / 𝑥]𝑦 = 𝑥 | |
2 | sbequ12 1729 | . . . . 5 ⊢ (𝑦 = 𝑥 → (𝜑 ↔ [𝑥 / 𝑦]𝜑)) | |
3 | 2 | bicomd 140 | . . . 4 ⊢ (𝑦 = 𝑥 → ([𝑥 / 𝑦]𝜑 ↔ 𝜑)) |
4 | 3 | sbimi 1722 | . . 3 ⊢ ([𝑦 / 𝑥]𝑦 = 𝑥 → [𝑦 / 𝑥]([𝑥 / 𝑦]𝜑 ↔ 𝜑)) |
5 | 1, 4 | ax-mp 5 | . 2 ⊢ [𝑦 / 𝑥]([𝑥 / 𝑦]𝜑 ↔ 𝜑) |
6 | sbbi 1910 | . 2 ⊢ ([𝑦 / 𝑥]([𝑥 / 𝑦]𝜑 ↔ 𝜑) ↔ ([𝑦 / 𝑥][𝑥 / 𝑦]𝜑 ↔ [𝑦 / 𝑥]𝜑)) | |
7 | 5, 6 | mpbi 144 | 1 ⊢ ([𝑦 / 𝑥][𝑥 / 𝑦]𝜑 ↔ [𝑦 / 𝑥]𝜑) |
Colors of variables: wff set class |
Syntax hints: ↔ wb 104 [wsb 1720 |
This theorem was proved from axioms: ax-mp 5 ax-1 6 ax-2 7 ax-ia1 105 ax-ia2 106 ax-ia3 107 ax-io 683 ax-5 1408 ax-7 1409 ax-gen 1410 ax-ie1 1454 ax-ie2 1455 ax-8 1467 ax-10 1468 ax-11 1469 ax-i12 1470 ax-4 1472 ax-17 1491 ax-i9 1495 ax-ial 1499 ax-i5r 1500 |
This theorem depends on definitions: df-bi 116 df-nf 1422 df-sb 1721 |
This theorem is referenced by: sbco3v 1920 |
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