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Proof step 3 from proof union-sizele (previous step)

#(s ∪ s0) ≤ # s ⊦ ∀ a. a ∈ s0 → a ∈ s

User applied rule cut formula

And obtained the following new goals

Proof step 4:
#(s ∪ s0) = # s, #(s ∪ s0) ≤ # s ⊦ ∀ a. a ∈ s0 → a ∈ s
Proof step 6:
#(s ∪ s0) ≤ # s ⊦ #(s ∪ s0) = # s, ∀ a. a ∈ s0 → a ∈ s

Rule argument

fma: #(s ∪ s0) = # s

Additional information

Used formulas: #(s ∪ s0) = # s