code: public LowerUpper addConstraint(final BigDecimal lowerLimit, final BigDecimal upperLimit, final int... assetIndeces) {
          return myConstraints.put(assetIndeces, new LowerUpper(lowerLimit, upperLimit));
      }
docs: Will add a constraint on the sum of the asset weights specified by the asset indices. Either (but not
  both) of the limits may be null.