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Proof step 2 from proof diff-del (previous step)

⊦ s1 \ s2 = (s1 -- a) \ s2, a ∈ s1 \ s2

Heuristic weak cut applied rule cut formula

And closed the goal!

Rule argument

fma: a ∈ s1

Additional information

Used simplifier rules: Difference: ⊦ a ∈ s1 \ s2 ↔ a ∈ s1 ∧ ¬ a ∈ s2;%diff-del-minus-3: ⊦ a ∈ s1 → (s -- a) \ s1 = s \ s1;%del-in: ⊦ ¬ a ∈ s → s -- a = s; Used formulas: a ∈ s1