A drive circuit comprising a DC bus configured to supply power to a load, a first fuel cell coupled to the DC bus and configured to provide a first power output to the DC bus, and a second fuel cell coupled to the DC bus and configured to provide a second power output to the DC bus supplemental to the first fuel cell. For each logical operator in an SQL server MEMO, in a top down manner from a root operator to the leaf operators, interesting distributions for the operators can be identified based on the properties of the operators. Specifically, first lens-group moves toward the object side, second lens-group moves along a path convex toward the image side, third lens-group monotonously moves only toward the object side, and fourth lens-group moves along a path convex toward the object side.