Fix function name typos in exercise 2.7 injectivity proof

[Copilot]

The injectivity proof in exercise 2.7 incorrectly referenced function f instead of g in two locations. The proof demonstrates that g: Γ_f → A defined by g(a,b) = a is a bijection, so the hypothesis and derivation should apply g, not f.

Changes

• Line 321: Changed hypothesis from f(a',b') = f(a'',b'') to g(a',b') = g(a'',b'')
• Line 322-323: Changed derivation from f(a',b') = a' = a'' = f(a'',b'') to g(a',b') = a' = a'' = g(a'',b'')

Before:

Let $(a',b'),(a'',b'')\in\Gamma_f$ such that $f(a',b') = f(a'',b'')$. For
contradiction, suppose that $(a',b')\neq (a'',b'')$. Since $f(a',b') = a' = a''
= f(a'',b'')$, it must be that $b'\neq b''$.

After:

Let $(a',b'),(a'',b'')\in\Gamma_f$ such that $g(a',b') = g(a'',b'')$. For
contradiction, suppose that $(a',b')\neq (a'',b'')$. Since $g(a',b') = a' = a''
= g(a'',b'')$, it must be that $b'\neq b''$.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>some typos in exercise 2.7</issue_title>
<issue_description> In the second paragraph, first several f should be g

</issue_description>

<agent_instructions>Fix typos in exercise 2.7</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes some typos in exercise 2.7 #26

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