C files listed as extensions.

pyproject.toml

 [build-system]
-requires = ["setuptools"]
+requires = ["setuptools", "cython"]
 build-backend = "setuptools.build_meta"
 
 [project]
@@ -47,3 +47,26 @@ dependencies = [
 #profile = "black"
 #skip_gitignore = true  # ignores files listed in .gitignore
 #skip_glob = ["/neuralmag/field_terms/code/*"]
+
+[tool.setuptools]
+ext-modules = [
+  {name = "tetrax.sample.mesh.fem_core.cythoncore",
+  sources = [
+      "tetrax/sample/mesh/fem_core/cythoncore.pyx",
+      "tetrax/sample/mesh/fem_core/dense2D_onthefly.c",
+      "tetrax/sample/mesh/fem_core/bessel_K1.c",
+      "tetrax/sample/mesh/fem_core/akmag.c",
+      "tetrax/sample/mesh/fem_core/rotation_mat.c",
+      "tetrax/sample/mesh/fem_core/matutils.c",
+      "tetrax/sample/mesh/fem_core/sorting.c",
+      "tetrax/sample/mesh/fem_core/utils.c",
+      "tetrax/sample/mesh/fem_core/bnd2d.c",
+      "tetrax/sample/mesh/fem_core/opmatspm.c",
+      "tetrax/sample/mesh/fem_core/galerkin.c",
+      "tetrax/sample/mesh/fem_core/fempreproc.c",
+      "tetrax/sample/mesh/fem_core/hotriang.c",
+      "tetrax/sample/mesh/fem_core/dense3D.c",
+      "tetrax/sample/mesh/fem_core/integration_of_functions.c"
+  ]
+  }
+]