diff -r 8990ac138cc2 -r 7c263b864371 tests/subfiles.py
--- a/tests/subfiles.py	Mon Jan 22 21:04:53 2018 +0200
+++ b/tests/subfiles.py	Mon Jan 22 21:21:53 2018 +0200
@@ -10,6 +10,10 @@
     library_standards.read_file(file)
 
 def determinant_test(model):
+    '''
+        Checks all subfile references for matrices with rows or columns all
+        zero.
+    '''
     yield from (
         error(subfile_reference, 'zero-determinant')
         for subfile_reference in model.subfile_references
@@ -46,6 +50,12 @@
 }
 
 def scaling_legality_test(model):
+    '''
+        Checks the part against primitive references with bad scaling. Some
+        primitives (e.g. pegs) are not allowed to be scaled in the
+        X or Z directions. Some (e.g. most studs) are not allowed to be scaled
+        in the Y direction either.
+    '''
     from fnmatch import fnmatch
     scaling_restrictions = library_standards['scaling restrictions']
     for subfile_reference in model.subfile_references: