Play with this app and build some proofs in natural deduction style.
Clicking on a formula automatically expands it with possible rules.
Assumptions are printed in blue. Selected one of them generates a new subtree.
Attaching two trees is done by dragging the root of the first one to the leaf of the second one.
If there only one possibility, this attachment is performed automatically.
The proof is done when the tree is surrounded by a red rectangle.

Have fun! Logic is fun