Given the question: Given the following passage  "Meditation was an aspect of the practice of the yogis in the centuries preceding the Buddha. The Buddha built upon the yogis' concern with introspection and developed their meditative techniques, but rejected their theories of liberation. In Buddhism, mindfulness and clear awareness are to be developed at all times; in pre-Buddhist yogic practices there is no such injunction. A yogi in the Brahmanical tradition is not to practice while defecating, for example, while a Buddhist monastic should do so.",  answer the following question. Note that the answer is present within the text.  Question: What part of the yogi's practices did the Buddha add to?
The answer is:
concern with introspection