Mrs. Doe and Mrs. Roe had babies at the same hospital at the same time. Mrs. Doe took home a girl and named her Nancy. Mrs. Roe received a boy and named him Larry. However, she was sure that she had a girl and brought a suit against the hospital. Blood tests showed that Mr. Roe was type 0, Mrs. Roe was type AB, Mr. Doe and Mrs. Doe were both type B, Nancy was type A and Larry was type O. Had an exchange occurred?