 According to Belnap and Green a true futurist theory should include the idea that at any moment of time – including any counterfactual moment – there is a true future, a so-called “thin red line” (Belnap and Green 1994), passing through that moment. Formally, this means that there must be a function, \(\TRL\), which gives the true future for any moment of time, \(t\). More precisely, \(\TRL(t)\) yields the linear past as well as the true future of \(t\), extended to a maximal  set.[5]