How can I include a pic figure as external file in groff?

While using groff and pic it is possible to do: $ cat test.ms .TL Test .NH 1 Test Header .PS circle arrow box .PE $ cat test.ms | groff -ms -p > test.ps Which generates a valid test.ps file containing the processed figure in pic. In larger documents, for mainteinability, it might be desirable to keep the picture pic description in a separate file and invoke such file from the main .ms file. While trying to get this working I have found that .so can source an external file, but I am finding issues to set up this configuration of embeding external .pic files into the main .ms file as shown here: $ cat test2.pic .PS circle arrow box .PE $ cat test2.ms .TL Test .NH 1 Test Header .so test2.pic $ cat test2.ms | groff -ms -p > test2.ps test2.pic:1: macro error: bad arguments to PS (not preprocessed with pic?) $ Which would be the way to achieve this inclusion of .pic code in the main macro .ms files as external files?