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Proof step 4 from proof del-rmdup-length-nle (previous step)

¬ ¬ a ∈ x, # rmdup(a ' + x) ≤ # rmdup(x -l a) ⊦

Heuristic simplifier applied rule simplifier

And obtained the following new goals

Proof step 5:
a ∈ x, # rmdup(x) ≤ # rmdup(x -l a) ⊦

Rule argument

Additional information

Used simplifier rules: a: ⊦ (x + y) + z = x + y + z; from specification list%rmdup-y: ⊦ a ∈ x → rmdup(a ' + x) = rmdup(x); from specification list-dup