This is an attempt at making a universal filter for the excellent SPICE software that is LTspice. A filter that can be both analog and digital, which can be configured to be either all-pass, low-pass, high-pass, band-pass or band-stop. A filter that can replace the slow process of calculating, implementing and then using a (multiple stage) filter, when the purpose of the simulation is different, not to mention the need to repeat the steps whenever changes occur. A filter that can be used anywhere a filtering function is needed, be it a quick example or a full-scale schematic, that's simple to use and reliable.
The result is filter.sub, the library holding the subcircuits, accompanied by its symbol, Filter.asy, made available under the GPL license. When browsing the .sub file (or any other SPICE library, model or netlist, for that matter), if Linux/UNIX is the environment used and if gEdit, or any other text editor using GtkSourceView (like Scribes or Mousepad) is the editor of choice, a useful tool can be this SPICE language file and, optional, the color settings, matching LTspice's.

©Vlad, 2008 - 2015