1)proof the following rule by using university database
sigma <condition1>(sigma < condition2>
(R)) = sigma
<condition2>
(sigma < condition1>
(R)) .
2)proof the following rule by using university database
sigma <cond1>(sigma < cond2>
(sigma <cond3>(R))
= sigma <cond1>
AND < cond2> AND < cond3>(R)))
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