From 05d53f2569d38dd4847af442d39fb35cb4592b16 Mon Sep 17 00:00:00 2001
From: SimonGuilloud <simon.guilloud@bluewin.ch>
Date: Fri, 28 Oct 2022 13:08:56 +0200
Subject: [PATCH] Exercises for FV (#89)

---
 lisa-examples/src/main/scala/Exercise.scala | 102 ++++++++++++++++++++
 1 file changed, 102 insertions(+)
 create mode 100644 lisa-examples/src/main/scala/Exercise.scala

diff --git a/lisa-examples/src/main/scala/Exercise.scala b/lisa-examples/src/main/scala/Exercise.scala
new file mode 100644
index 00000000..89e66734
--- /dev/null
+++ b/lisa-examples/src/main/scala/Exercise.scala
@@ -0,0 +1,102 @@
+import lisa.automation.kernel.SimplePropositionalSolver.*
+import lisa.automation.kernel.SimpleSimplifier.*
+import lisa.utils.Printer
+
+object Exercise extends lisa.Main {
+
+  private val x = VariableLabel("x")
+  private val y = VariableLabel("y")
+  private val z = VariableLabel("z")
+  private val f = SchematicFunctionLabel("f", 1)
+  private val P = SchematicPredicateLabel("P", 1)
+  private val Q = SchematicPredicateLabel("Q", 2)
+  private val H = VariableFormulaLabel("H")
+
+  /////////////////////////
+  // Propositional Logic //
+  /////////////////////////
+
+  THEOREM("double_negation_elim") of "|- 'a ⇔ ¬¬'a" PROOF {
+    val hyp = have("'a |- 'a") by Hypothesis
+
+    andThen("|- !'a; 'a") by RightNot
+    andThen("!!'a |- 'a") by LeftNot
+    val direction1 = andThen("|- !!'a ==> 'a") by RightImplies
+
+    have("'a; !'a |- ") by LeftNot(hyp)
+    andThen(("'a |- !!'a")) by RightNot
+    val direction2 = andThen("|- !!'a ==> 'a") by RightImplies
+
+    have("|- !!'a <=> 'a") by RightIff(direction1, direction2)
+    showCurrentProof()
+  }
+  show
+
+  /*
+  THEOREM("pierce-law") of "|- ((('A ==> 'B) ==> 'A) ==> 'A)" PROOF {
+    ???
+  }
+  show
+   */
+
+  ///////////////////////
+  // First Order Logic //
+  ///////////////////////
+
+  /*
+  THEOREM("russel_Paradox") of "∀'x. 'Q('y, 'x) ↔ ¬'Q('x, 'x) ⊢" PROOF {
+    have("'Q('y, 'y) <=> !'Q('y, 'y) |- ") by Restate
+    andThen("∀'x. 'Q('y, 'x) <=> !'Q('x, 'x) |- ") by LeftForall(y)
+  }
+  show
+
+  THEOREM("all-comm") of "|- (∀'x. ∀'y. 'Q('x, 'y)) ⇔ (∀'y. ∀'x. 'Q('x, 'y))" PROOF {
+    val base = have("'Q('x, 'y) |- 'Q('x, 'y)") by Hypothesis
+
+    andThen ("∀'y. 'Q('x, 'y) |- 'Q('x, 'y)") by LeftForall(y)
+    andThen ("∀'x. ∀'y. 'Q('x, 'y) |- 'Q('x, 'y)") by LeftForall(x)
+    andThen ("∀'x. ∀'y. 'Q('x, 'y) |- ∀'x. 'Q('x, 'y)") by RightForall()
+    andThen ("∀'x. ∀'y. 'Q('x, 'y) |- ∀'y. ∀'x. 'Q('x, 'y)") by RightForall()
+    val direction1 = andThen ("|- (∀'x. ∀'y. 'Q('x, 'y)) ==> (∀'y. ∀'x. 'Q('x, 'y))") by Restate
+
+    have ("∀'x. 'Q('x, 'y) |- 'Q('x, 'y)") by LeftForall(x)(base)
+    andThen ("∀'y. ∀'x. 'Q('x, 'y) |- 'Q('x, 'y)") by LeftForall(y)
+    andThen ("∀'y. ∀'x. 'Q('x, 'y) |- ∀'y. 'Q('x, 'y)") by RightForall()
+    andThen ("∀'y. ∀'x. 'Q('x, 'y) |- ∀'x. ∀'y. 'Q('x, 'y)") by RightForall()
+    val direction2 = andThen ("|- (∀'y. ∀'x. 'Q('x, 'y)) ==> (∀'x. ∀'y. 'Q('x, 'y))") by Restate
+    have ("|- (∀'x. ∀'y. 'Q('x, 'y)) <=> (∀'y. ∀'x. 'Q('x, 'y))") by RightIff(direction1, direction2)
+  }
+  show
+   */
+
+  /*
+  THEOREM("fixed_Point_Double_Application") of "∀'x. 'P('x) ⇒ 'P('f('x)) ⊢ 'P('x) ⇒ 'P('f('f('x)))" PROOF {
+    ???
+  }
+  show
+   */
+
+  ////////////////////
+  //   Set Theory   //
+  ////////////////////
+
+  /*
+  THEOREM("SubsetRefl") of " ⊢ subset_of('x, 'x)" PROOF {
+    have(subsetAxiom)
+    val r1 = andThen(() |- forall(z, in(z, x) ==> in(z, x)) <=> subset(x, x)) by InstantiateForallMultipleWithoutFormula(x, x)
+    have(() |- in(z, x) ==> in(z, x)) by Restate
+    andThen(() |- forall(z, in(z, x) ==> in(z, x))) by RightForall
+    andThen(applySubst(forall(z, in(z, x) ==> in(z, x)) <=> subset(x, x)))
+    Discharge(r1)
+  }
+  show
+   */
+
+  /*
+  THEOREM("SubsetRefl") of " subset_of('x, 'y); subset_of('y, 'z) ⊢ subset_of('x, 'z)" PROOF {
+   ???
+  }
+  show
+   */
+
+}
-- 
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