in test/modules/dfem_diffusion/d_diffusion_2d_3c_l2_error_mms.lua, we had a method SimTest_IP_MMS_L2error() that was useful to do manufactured solutions and check convergence rates. I wrote it and use it to found a bug on the IP implementation. If I remember correctly, this was an example of using the software as a library and write one's little code with the library. We didn't want to have manufactured solution (MS) as part of the code itself, but we were happy to test MS on the side. I would appreciate if we could resurrect this. I could even use it as a tutorial on how to write a new code, using opensn as a library.