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• Performing a fuzzy search may retrieve terms with similar spellings: (( a robust _. heilmann book ) or ( a robust e. (heimann OR heinemann) eebooks ))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (( a robust _. heilmann book ) OR ( a robust e. (heimann OR heinemann) eebooks )).
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( a robust _. heilmann book ) or ( a robust e. (heimann OR heinemann) eebooks ))".
• Adding a wildcard symbol may retrieve word variants: (( a robust _. heilmann book ) or ( a robust e. (heimann OR heinemann) eebooks ))*.