Check for existence of the include path before including it in divided_difference.m.
-rw-r--r-- 936 divided_difference.m
-rw-r--r-- 170 errcheck.m
-rw-r--r-- 436 erreval.m
-rwxr-xr-x 315 run-tests.m
-rw-r--r-- 1404 unit_init.m
-rw-r--r-- 783 unit_results.m
-rw-r--r-- 2194 unit_test.m
-rw-r--r-- 169 unit_test_equals.m
-rw-r--r-- 166 unit_test_err.m