a layout should contain one and only reference to "_main" -- there should be just one main slot on the page. The obvious case where you'd want an exception is to refer to the name of the main card somewhere on the sidebar, like {{_main|name}}. I haven't tested this, but I think that should still be ok so long as we don't use the exact name "_main" but something else with the same key, like "_MAIN" or "_main#$%". Once we have validations, we should probably check this whenever someone saves a layout card. --Ethan

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