Publisher‘s note. We very much regret that in the final stages of production an error was introduced into the title of the above article. In attempting to clarify the meaning of “P” in the term “P controller”, the parenthetical phrase “(proportional)” was accidentally introduced into the wrong location, giving the meaningless phrase printed in the original title. We wish to apologise to authors and readers for this error and any confusion it may have caused: the correct version of the title is as given below:

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