cf. lde'u chos 'byung rgyas pa [1987] (pp. 256-261)
an overview of the administrative ཡུལ་སྡེ། (or ཡུལ་དཔོན་ཙན།) districts of the Four Horns acc. to lde'u chos 'byung rgyas pa [1987] and padma bka' thang [mst] (i.e. རྒྱལ་པོའི་བཀའ་ཐང་། ) is found in Dotson & Hazod 2009 (p. 209)
on the term ཡུལ་སྡེ།, see Dotson & Hazod 2009 (p. 261)