AIMA.Core.Logic.FOL.KB.Data.Chain.getProofStep C# (CSharp) Method

        public ProofStep getProofStep()
        {
            if (null == proofStep)
            {
                // Assume was a premise
                proofStep = new ProofStepPremise(this);
            }
            return proofStep;
        }

 public ProofStepChainContrapositive(Chain contrapositive,
         Chain contrapositiveOf)
 {
     this.contrapositive = contrapositive;
     this.contrapositiveOf = contrapositiveOf;
     this.predecessors.Add(contrapositiveOf.getProofStep());
 }