The Laths were the owners of a farm that they wanted to sell. Mrs. Mitchell considered purchasing the land but found that an ice house located across the road was objectionable. Mitchell argued that the Laths orally agreed to remove the ice house in consideration of her promise to purchase the property, which she agreed to purchase for $8,400 in a written contract. After Mitchell moved into her new home and made several improvements to the land, the Laths never removed the ice house and expressly communicated their intention not to do so. Mitchell sued the Laths for breach of contract. What effect does the parol evidence rule have on the admissibility of the oral agreement to remove the ice house? Why this effect?