code: public static final DoubleMatrix2D add(DoubleMatrix2D A, DoubleMatrix2D B){
  		if(A.rows()!=B.rows() || A.columns()!=B.columns()){
  			throw new IllegalArgumentException("wrong matrices dimensions");
  		}
  		DoubleMatrix2D ret = DoubleFactory2D.dense.make(A.rows(), A.columns());
  		for(int i=0; i<ret.rows(); i++){
  			for(int j=0; j<ret.columns(); j++){
  				ret.setQuick(i, j, A.getQuick(i, j) + B.getQuick(i, j));
  			}
  		}
  
  		return ret;
  	}
docs: Returns C = A + B.
  Useful in avoiding the need of the copy() in the colt api.