Diego Rivera. Frida Kahlo. A new exhibition at the Nelson Atkins Museum of Art. It feels like old ground, and yet we (and, by we, I mean museums and galleries), keep on putting new exhibitions up, re-showing the same work over and over again. Isn’t this just a curriculum which we are repeating? A monopoly set where we never get to pass ‘GO’? A dream that we never seem to be able to wake up from? No, actually it isn’t. What we can learn from Frida and Diego, and all other artists who are being exhibited (be it once, twice, or two thousand times), is that their work is important.…