first iteration of __isub__ method in RankingStat

gstlal-inspiral/python/far.py

@@ -257,6 +257,15 @@ class RankingStat(snglcoinc.LnLikelihoodRatioMixin):
 		self.allow_impossible_candidates |= other.allow_impossible_candidates
 		return self
 
+	def __isub__(self, other):
+		if type(other) != type(self):
+			raise TypeError(other)
+		self.numerator -= other.numerator
+		self.denominator -= other.denominator
+		self.zerolag -= other.zerolag
+		self.allow_impossible_candidates |= other.allow_impossible_candidates
+		return self
+
 	def copy(self):
 		new = type(self)(template_ids = self.template_ids, instruments = self.instruments, population_model_file = self.population_model_file, dtdphi_file = self.dtdphi_file, min_instruments = self.min_instruments, delta_t = self.delta_t)
 		new.numerator = self.numerator.copy()