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• If you are trying to use Boolean operators, they must be ALL CAPS: (( gail robert e. hermann books ) OR ( ((a robust) OR (_ robust)) b. heimann ebook ))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( gail robert e. hermann books ) or ( ((a robust) OR (_ robust)) b. heimann ebook ))~".
• Adding a wildcard symbol may retrieve word variants: (( gail robert e. hermann books ) or ( ((a robust) OR (_ robust)) b. heimann ebook ))*.