An excerpt from a book by Mark Epstein called Thoughts without a Thinker :

In the early days of my interest in Buddhism and psychology,I was given a particularly vivid demonstation of how difficult it was going to be to forge an integration between the two.  Some friends of mine had arranged for an encounter between two prominent visiting Buddhist teachers at the house of a Harvard University psychology professor. These were teachers from two distinctly different Buddhist traditions who had never met and whose traditions had in fact had very little contact over the past thousand years.  Before the worlds of Buddhism and Western psychology could come together, the various strands of Buddhism would have to encounter one another.  We were to witness the first such dialogue.

The teachers, seventy-year-old Kalu Rinpoche of Tibet, a veteran of years of solitary retreat, and the Zen master Seung Sahn, the first Korean Zen master to teach in the United States, were to test each other's understanding of the Buddha's teachings for the benefit of the onlooking Western students.  This was to be a high form of what was being called dharma combat (the clashing of great minds sharpened by years of study and meditation), and we were waiting with all the anticipation that such a historic encounter deserved.  The two monks entered with swirling robes -- maroon and yellow for the Tibetan, austere grey and black for  the Korean -- and were followed by retinues of younger monks and translators with shaven heads.  They settled onto cushions in the familiar cross-legged positions, and the host made it clear that the younger Zen master was to begin.  The Tibetan lama sat very still, fingering a wooden rosary (mala) with one hand while murmuring "Om mani padme hum" continuously under his breath.

The Mumonkan, also known as "The Gateless Gate", is a classic Zen text consisting of 48 koans with commentaries and verses collected by Wumen Hui-k'ai (Mumon Ekai), a Chinese Zen master who lived in the 13th century Song period.

The Great Way has no gate,
A thousand roads enter it.
When one passes through this gateless gate,
He freely walks between heaven and earth.