Bug fix

Incorrectly added to ldiff rather than subtract.

 27.891316086 seconds (2062424 bytes allocated)
elapsed time: 3.537040726 seconds (5680 bytes allocated)
elapsed time: 2.435748415 seconds (5680 bytes allocated)
elapsed time: 2.336278089 seconds (5680 bytes allocated)
elapsed time: 2.236659681 seconds (5680 bytes allocated)
elapsed time: 2.169905695 seconds (5680 bytes allocated)
elapsed time: 2.131642029 seconds (5680 bytes allocated)
elapsed time: 2.086907439 seconds (5680 bytes allocated)
elapsed time: 2.0515717 seconds (5680 bytes allocated)
elapsed time: 2.017587282 seconds (5680 bytes allocated)