You wrote that:<p><pre><code>  forall f, x: f(x) = f(y) implies x = y
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What if f(x) is x^2 and f(y) is y^2? You could have x=2 and y=-2.<p>I&#x27;m sure there is something I&#x27;m missing, but wouldn&#x27;t that make the proposition false? i.e. f(x) = f(y) is true, but x = y is false, thus true implies false is false