nbgrader:  do not overwrite manual nbgrader's when re-running nbgrader
nbgrader:  do not overwrite manual nbgrader's when re-running nbgrader

tscholl2/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc