Hung

"Hung is the heart syllable, the seed syllable of Padmasambhava. Hung is also the seed syllable of Yeshé Tsogyel. Hung is the primordial union of emptiness and form; the understanding of which, in itself, encompasses refuge and bodhicitta. There is no refuge and bodhicitta greater than the comprehension of the non-duality of emptiness and form."

Seven Line Song, Ngak'chang Rinpoche, Aro Encyclopaedia