“…This phenomenon, which does not appear in the classical theory of local fields, is known as ferocious ramification [82], and has been the major source of trouble in developing ramification theory for higher dimensional local fields, or more generally for complete discrete valuation fields with imperfect residue field. Various approaches and contributions to the theory, in chronological order, are due to K. Kato [33,34,35], O. Hyodo [27], I. Fesenko [14], I. Zhukov [81,82], A. Abbes and T. Saito [1] [2], and J. Borger [7,6].…”