def find(self, tree):
# We keep coefficients and spatial derivatives in separate lists
# because we need separate criteria to uniqify the lists.
self._coefficients = []
self._spatialDerivatives = []
self.visit(tree)
# Coefficients define __eq__ and __hash__ so the straight uniqify works.
# For spatial derivatives, we need to compare the coefficients.
coefficients = uniqify(self._coefficients)
spatialDerivatives = uniqify(self._spatialDerivatives, lambda x: x.operands()[0])
return coefficients, spatialDerivatives