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All the chapters are very rich in content : sometimes this richness makes the argumentation slightly difficult to follow , although everything is made clear in the end and all the different considerations find their exact place in the puzzle . But although it is very difficult to stop , we 'll keep on trying with relief wells . This concept is sometimes difficult to understand , since there is typically a finite sum the company considers " at risk " when they sign a contract with an SEO firm or commit internal resources to the task . ( 3 ) Ubicomp systems are still very difficult to build , and it is rare for them to be used outside of a laboratory setting . The sodium content of diets is notably difficult to measure , but there is little doubt that Americans consume more sodium than they need [ 30 ] .