add Guido's reasoning behind running all tests twice
diff --git a/Makefile.pre.in b/Makefile.pre.in
index 207798e..843c50f 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -514,6 +514,13 @@
######################################################################
# Test the interpreter (twice, once without .pyc files, once with)
+# In the past, we've had problems where bugs in the marshalling or
+# elsewhere caused bytecode read from .pyc files to behave differently
+# than bytecode generated directly from a .py source file. Sometimes
+# the bytecode read from a .pyc file had the bug, somtimes the directly
+# generated bytecode. This is sometimes a very shy bug needing a lot of
+# sample data.
+
TESTOPTS= -l
TESTPROG= $(srcdir)/Lib/test/regrtest.py
TESTPYTHON= $(RUNSHARED) ./$(BUILDPYTHON) -E -tt