Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2015/digital-systems-linear-algebra-introduction-to-formal-proof/7/a/ii/solution

A common-context sequent version of the disjunction elimination rule is
The second and third premises record exactly the two boxed subproofs of the natural deduction presentation. Their additional case assumptions do not occur in the conclusion: that is assumption discharge. The surrounding context remains throughout. A formulation with different surrounding contexts is equivalent after retaining their union by weakening.

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