Proof: Addition Theorem
Let's prove the following theorem:
a + a = a ⋅ 2
    
    
    
    Proof:
| # | Claim | Reason | 
|---|---|---|
| 1 | a ⋅ 1 = a | a ⋅ 1 = a | 
| 2 | (a ⋅ 1) + a = a + a | if a ⋅ 1 = a, then (a ⋅ 1) + a = a + a | 
| 3 | a + a = (a ⋅ 1) + a | if (a ⋅ 1) + a = a + a, then a + a = (a ⋅ 1) + a | 
| 4 | a = a ⋅ 1 | a = a ⋅ 1 | 
| 5 | a + a = (a ⋅ 1) + (a ⋅ 1) | if a + a = (a ⋅ 1) + a and a = a ⋅ 1, then a + a = (a ⋅ 1) + (a ⋅ 1) | 
| 6 | (a ⋅ 1) + (a ⋅ 1) = a ⋅ (1 + 1) | (a ⋅ 1) + (a ⋅ 1) = a ⋅ (1 + 1) | 
| 7 | 1 + 1 = 2 | 1 + 1 = 2 | 
| 8 | a ⋅ (1 + 1) = a ⋅ 2 | if 1 + 1 = 2, then a ⋅ (1 + 1) = a ⋅ 2 | 
| 9 | (a ⋅ 1) + (a ⋅ 1) = a ⋅ 2 | if (a ⋅ 1) + (a ⋅ 1) = a ⋅ (1 + 1) and a ⋅ (1 + 1) = a ⋅ 2, then (a ⋅ 1) + (a ⋅ 1) = a ⋅ 2 | 
| 10 | a + a = a ⋅ 2 | if a + a = (a ⋅ 1) + (a ⋅ 1) and (a ⋅ 1) + (a ⋅ 1) = a ⋅ 2, then a + a = a ⋅ 2 | 
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