Responsa for Bava Batra 306:7

וחכמים אומרים המוציא מחבירו עליו הראיה וכו':

it is regarded as a private domain<span class="x" onmousemove="('comment',' Because the public abstain from using it on account of its growing crops. ');">²⁰</span> in both respects.<span class="x" onmousemove="('comment',' Lit., 'to here and to here'; as regards the Sabbath (v. supra p. 665, n. 15), and as regards 'doubtful Levitical uncleanness' which in a private domain is regarded as unclean. Consequently. if a person entered the valley and is not certain whether he entered it in summer or in winter he should, according to R. Nathan, be regarded as clean if his case was dealt with by the court in the summer, and as unclean if dealt with in the winter. According to R. Jacob, who does not take into consideration the time the decision is given, the person would always be regarded as clean whatever the season in which his case is dealt with (since a person is presumed to be usually clean), unless witnesses testified that they saw him enter the valley in winter. ');">²¹</span> Raba said: This<span class="x" onmousemove="('comment',' That a walled valley in the summer season is subject to the laws of a public domain in respect of Levitical uncleanness. ');">²²</span> has reference only<span class="x" onmousemove="('comment',' Lit., 'they did not teach but', ');">²³</span> [to the case] where a winter has not passed over it,<span class="x" onmousemove="('comment',' Since the time when a wall was put round it. ');">²⁴</span> but [where] a winter has passed over it, [it is regarded as] a private domain in all respects.<span class="x" onmousemove="('comment',' Even in the summer season. Once it has acquired the status of a private domain it retains that status permanently. ');">²⁵</span> THE SAGES, HOWEVER, SAY: HE WHO CLAIMS FROM THE OTHER HAS TO PRODUCE THE PROOF.

Teshuvot Maharam