Merge pull request #8 from SimonGuilloud/main

src/main/scala/lisa/KernelHelpers.scala

src/main/scala/lisa/kernel/Printer.scala

src/main/scala/lisa/kernel/fol/FOL.scala

src/main/scala/lisa/kernel/fol/FormulaDefinitions.scala

src/main/scala/lisa/kernel/fol/Substitutions.scala

src/main/scala/lisa/kernel/fol/TermDefinitions.scala

src/main/scala/lisa/kernel/proof/Judgement.scala

src/main/scala/lisa/kernel/proof/SCProofChecker.scala

src/main/scala/lisa/kernel/proof/SequentCalculus.scala

src/main/scala/proven/DSetTheory/Part1.scala

src/main/scala/proven/ElementsOfSetTheory.scala

src/test/scala/lisa/kernel/IncorrectProofsTests.scala

src/test/scala/lisa/kernel/ProofTests.scala

src/test/scala/lisa/proven/ElementsOfSetTheoryTests.scala