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the input build to be specified, as well as constraints on the inputs of the inputs build. For instance, you can require that a build has input `system = "i686-linux"'. This is important when one binary build serves as an input to another binary build. Obviously, we shouldn't pass a build on i686-linux as an input to another on i686-darwin. Hence the necessity for constraint. The constraint are currently quite limited. What you really want to say is that the "system" input of the other build has to match the "system" input of this build. But those require a bit more work since they introduce dependencies between inputs.