In City of Mission v. Popplewell, 156 Tex. 269, 294 S.W.2d 712 (Tex. 1956), the City of Mission had the burden of proving its title to a disputed alley. Although the supreme court recognized that testimony of the expert engineer could potentially bridge the gap in the chain of title, it held that the city was required to proved its legal title by valid written instruments, not by oral evidence: "Parol evidence in the form of opinions and conclusions without documentary basis is inadmissible to establish such title, and even if admitted without objection is of no probative force." (156 Tex. at 277.) In Popplewell, the issue was not whether extrinsic evidence could be used to prove the contents of an unambiguous document. Rather, the issue was whether testimonial evidence, in the form of opinions and conclusions, could--standing alone--establish record title in a trespass to try title action. Popplewell, 156 Tex. at 277. The court held that it could not. Id.