= Exactness of localization
For a <multiplicative subset> $S$ of a <commutative ring> $A$, localization preserves <short exact sequences> of $A$-modules. Surjectivity follows by lifting a numerator. If a fraction maps to zero, some element of $S$ annihilates its image, so multiplying its numerator by that element puts it into the original kernel. For an injective original map, the same criterion shows its localized kernel is zero. Thus $S^{-1}A$ is a <flat module> over $A$.
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