I’ve added a new section “Double filtrators” to the book “Algebraic General Topology. Volume 1”.

I show that it’s possible to describe $latex (\mathsf{FCD})$, $latex (\mathsf{RLD})_{\mathrm{out}}$, and $latex (\mathsf{RLD})_{\mathrm{in}}$ entirely in terms of filtrators (order). This seems not to lead to really interesting results but it’s curious.