9:41 AM
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There is some sort of symmetry in the definition of flat module and pure short exact sequence which can be made precise as follows. Let $R$ be a ring (with unit), $\mathcal{R}$ be the class of all right $R$-modules, and $\mathcal{S}$ be the class of all short exact sequences of left $R$-modules. ...

Hope someone could take a look at this question and offer some thoughts. Much appreciated!