 Complete the following phrase:  According to the Convention of Calcutta of 189094 signed by Great Britain and Qing dynasty China, the market at Dromo was opened to India. At that time there was a wall-like structure across the valleys stream extending part way up each side of the valley thus blocking the road to the Tibetan capital, Lhasa. This was the demarcation line that British subjects were forbidden to cross and was manned by 20 Tibetan soldiers under a sergeant along with three Chinese officials. The construction of the wall was reported to be one of the incidents that led to the 