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This permits rapid multiplexing of the different assemblies because coincidence can be evaluated within short time intervals , as it does not require temporal summation . The semantics of a rule is that at the time an individual instance is accessed , updated , inserted or deleted , there is a current instance ( for retrieves , updates and deletes ) and a new instance ( for updates and appends ) . In addition to his primary duties , DeWitt is pursuing his Ph.D. in Computer Science at the University of Southern California , is the Officer-in-Charge of the Air Base Honor Guard , and is a senior scout leader in the community . Although stimuli of high spatial frequency become unresolvable at saccadic speeds , stimuli of low spatial frequency can be seen only in motion but become considerably more visible at saccadic speeds ( Burr and Ross , 1982 ) . Civil engineers design solutions to cope with many of our planet 's most serious problems - - foul air ; decaying cities , roadways , and bridges ; clogged airports and highways ; polluted streams , rivers , and lakes .