code: @SuppressWarnings({"WeakerAccess", "unused"}) // For library users
      @Nullable
      public NumericRefinement getNumericRefinement(@NonNull String attribute, int operator) {
          NumericRefinement.checkOperatorIsValid(operator);
          final SparseArray<NumericRefinement> attributeRefinements = numericRefinements.get(attribute);
          return attributeRefinements == null ? null : attributeRefinements.get(operator);
      }
docs: Gets the current numeric refinement for an attribute and an operator.
  
  @param attribute the attribute to refine on.
  @param operator  one of the {@link NumericRefinement#OPERATOR_EQ operators} defined in {@link NumericRefinement}.
  @return a {@link NumericRefinement} describing the current refinement for these parameters, or {@code null} if there is none.