lean2/tests/lean/run/cody2.lean

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import logic
open eq
definition subsets (P : Type) := P → Prop.
section
hypothesis A : Type.
hypothesis r : A → subsets A.
hypothesis i : subsets A → A.
hypothesis retract {P : subsets A} {a : A} : r (i P) a = P a.
definition delta (a:A) : Prop := ¬ (r a a).
local notation `δ` := delta.
theorem delta_aux : ¬ (δ (i delta))
:= assume H : δ (i delta),
H (subst (symm retract) H).
check delta_aux.
end