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diff --git a/helm/software/matita/library/didactic/exercises/natural_deduction1.ma b/helm/software/matita/library/didactic/exercises/natural_deduction1.ma
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+(* Istruzioni: 
+
+     http://mowgli.cs.unibo.it/~tassi/exercise-natural_deduction.html 
+
+*)
+
+(* Esercizio 0
+   ===========
+
+   Compilare i seguenti campi:
+
+   Nome1: ...
+   Cognome1: ...
+   Matricola1: ...
+   Account1: ...
+
+   Nome2: ...
+   Cognome2: ...
+   Matricola2: ...
+   Account2: ...
+
+*)
+
+include "didactic/support/natural_deduction.ma".
+
+theorem EM: ∀A:CProp. A ∨ ¬ A.
+assume A: CProp.
+apply rule (prove (A ∨ ¬A));
+apply rule (RAA [H] (⊥)).
+apply rule (¬_e (¬(A ∨ ¬A)) (A ∨ ¬A));
+       [ apply rule (discharge [H]).
+       | apply rule (⊥_e (⊥));
+         apply rule (¬_e (¬¬A) (¬A));
+          [ apply rule (¬_i [K] (⊥)).
+       apply rule (¬_e (¬(A ∨ ¬A)) (A ∨ ¬A));
+             [ apply rule (discharge [H]).
+             | apply rule (∨_i_r (¬A)).
+          apply rule (discharge [K]).
+             ]
+          | apply rule (¬_i [K] (⊥)).
+       apply rule (¬_e (¬(A ∨ ¬A)) (A ∨ ¬A));
+             [ apply rule (discharge [H]).
+             | apply rule (∨_i_l (A)).
+          apply rule (discharge [K]).
+             ]
+          ]
+       ]
+qed.
+
+theorem ex0 : (F∧¬G ⇒ L ⇒ E) ⇒ (G ∧ L ⇒ E) ⇒ F ∧ L ⇒ E.
+apply rule (prove ((F∧¬G ⇒ L ⇒ E) ⇒ (G ∧ L ⇒ E) ⇒ F ∧ L ⇒ E));
+(*BEGIN*)
+apply rule (⇒_i [h1] ((G ∧ L ⇒ E) ⇒ F ∧ L ⇒ E));
+apply rule (⇒_i [h2] (F ∧ L ⇒ E));
+apply rule (⇒_i [h3] (E));
+apply rule (∨_e (G∨¬G) [h4] (E) [h4] (E));
+       [ apply rule (lem EM);
+       | apply rule (⇒_e (G∧L⇒E) (G∧L));
+           [ apply rule (discharge [h2]);
+           | apply rule (∧_i (G) (L));
+               [ apply rule (discharge [h4]);
+               | apply rule (∧_e_r (F∧L));
+                 apply rule (discharge [h3]);
+               ]
+           ]
+       | apply rule (⇒_e (L⇒E) (L));
+           [ apply rule (⇒_e (F∧¬G ⇒ L ⇒ E) (F∧¬G));
+               [ apply rule (discharge [h1]);
+               | apply rule (∧_i (F) (¬G));
+                   [ apply rule (∧_e_l (F∧L));
+                     apply rule (discharge [h3]);
+                   | apply rule (discharge [h4]);
+                   ]
+               ]  
+           | apply rule (∧_e_r (F∧L));
+             apply rule (discharge [h3]);
+           ]
+       ]
+(*END*)
+qed.
+
+theorem ex1 : (F ⇒ G∨E) ⇒ (G ⇒ ¬ (L ∧ ¬E)) ⇒ (¬L ∨ F) ⇒ L ⇒ E.
+apply rule (prove (F ⇒ G∨E) ⇒ (G ⇒ ¬ (L ∧ ¬E)) ⇒ (¬L ∨ F) ⇒ L ⇒ E);
+(*BEGIN*)
+apply rule (⇒_i [h1] ((G ⇒ ¬ (L ∧ ¬E)) ⇒ (¬L ∨ F) ⇒ L ⇒ E));
+apply rule (⇒_i [h2] ((¬L ∨ F) ⇒ L ⇒ E));
+apply rule (⇒_i [h3] (L ⇒ E));
+apply rule (⇒_i [h4] (E));
+apply rule (∨_e (¬L∨F) [h5] (E) [h5] (E));
+       [ apply rule (discharge [h3]);
+       | apply rule (⊥_e (⊥));
+    apply rule (¬_e (¬L) (L));
+           [ apply rule (discharge [h5]);
+           | apply rule (discharge [h4]);
+           ]
+       | apply rule (∨_e (G∨E) [h6] (E) [h6] (E));
+          [ apply rule (⇒_e (F⇒G∨E) (F));
+             [ apply rule (discharge [h1]);
+             | apply rule (discharge [h5]);
+             ]
+          | apply rule (RAA [h7] (⊥));
+       apply rule (¬_e (¬ (L ∧ ¬E)) (L ∧ ¬E));
+              [ apply rule (⇒_e (G ⇒ ¬ (L ∧ ¬E)) (G));
+                  [apply rule (discharge [h2]);
+                  |apply rule (discharge [h6]);
+                  ]
+              | apply rule (∧_i (L) (¬E));
+                  [ apply rule (discharge [h4]);
+                  | apply rule (discharge [h7]);
+                  ]
+              ]
+          | apply rule (discharge [h6]);
+          ]
+       ]
+(*END*)
+qed.
+
+theorem ex2 : (F∧G ⇒ E) ⇒ (H ⇒ E∨F) ⇒ (¬G ⇒ ¬H) ⇒ H ⇒ E.
+apply rule (prove ((F∧G ⇒ E) ⇒ (H ⇒ E∨F) ⇒ (¬G ⇒ ¬H) ⇒ H ⇒ E));
+(*BEGIN*)
+apply rule (⇒_i [h1] ((H ⇒ E∨F) ⇒ (¬G ⇒ ¬H) ⇒ H ⇒ E));
+apply rule (⇒_i [h2] ((¬G ⇒ ¬H) ⇒ H ⇒ E));
+apply rule (⇒_i [h3] (H ⇒ E));
+apply rule (⇒_i [h4] (E));
+apply rule (∨_e (G∨¬G) [h5] (E) [h5] (E));
+       [apply rule (lem EM);
+       | apply rule (∨_e (E∨F) [h6] (E) [h6] (E));
+          [ apply rule (⇒_e (H ⇒ E∨F) (H));
+              [ apply rule (discharge [h2]);
+              | apply rule (discharge [h4]);
+              ]
+          | apply rule (discharge [h6]);
+          | apply rule (⇒_e (F∧G ⇒ E) (F∧G));
+             [apply rule (discharge [h1]);
+             |apply rule (∧_i (F) (G));
+               [ apply rule (discharge [h6]);
+               | apply rule (discharge [h5]);
+               ]
+             ]
+          ]
+       | apply rule (⊥_e (⊥));
+    apply rule (¬_e (¬H) (H));
+       [ apply rule (⇒_e (¬G ⇒ ¬H) (¬G));
+              [ apply rule (discharge [h3]);
+              | apply rule (discharge [h5]);
+              ]
+           | apply rule (discharge [h4]);
+           ]
+       ]
+(*END*)
+qed.
+
+theorem ex3 : (F∧G ⇒ E) ⇒ (¬E ⇒ G∨¬L) ⇒ (L ⇒ F) ⇒ L ⇒ E.
+apply rule (prove ((F∧G ⇒ E) ⇒ (¬E ⇒ G∨¬L) ⇒ (L ⇒ F) ⇒ L ⇒ E)).
+(*BEGIN*)
+apply rule (⇒_i [h1] ((¬E ⇒ G∨¬L) ⇒ (L ⇒ F) ⇒ L ⇒ E));
+apply rule (⇒_i [h2] ((L ⇒ F) ⇒ L ⇒ E));
+apply rule (⇒_i [h3] (L ⇒ E));
+apply rule (⇒_i [h4] (E));
+apply rule (RAA [h5] (⊥));
+apply rule (∨_e (G∨¬L) [h6] (⊥) [h6] (⊥));
+       [ apply rule (⇒_e (¬E ⇒ G∨¬L) (¬E));
+           [ apply rule (discharge [h2]);
+           | apply rule (discharge [h5]);
+           ]
+       | apply rule (¬_e (¬E) (E));
+       [ apply rule (discharge [h5]);
+           | apply rule (⇒_e (F∧G ⇒ E) (F∧G));
+              [ apply rule (discharge [h1]);
+              | apply rule (∧_i (F) (G));
+                  [ apply rule (⇒_e (L⇒F) (L));
+                     [ apply rule (discharge [h3]);
+                     | apply rule (discharge [h4]);
+                     ]
+                  | apply rule (discharge [h6]);
+                  ]
+              ]
+           ]
+       | apply rule (¬_e (¬L) (L));
+           [ apply rule (discharge [h6]);
+           | apply rule (discharge [h4]);
+           ]
+       ]
+(*END*)
+qed.
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