What is the phonetic realization of a sequence of "voiced-voiceless" or "voiceless-voiced" obstruents (especially sibilants) of the same place of articulation? For example, /zs/ as in "Mrs. Stratford", /sz/ as in "cross zebra", /ʒʃ/ as in "louge shop", /td/ as in "Bret Dawson", etc. Is there assimilation? If so, is the first or the second element assimilated with the other (/zs/ -> [zz] or [ss])? Excuse the artificial examples because I can't think of anything better at the moment. Also, "subpoena" is typically transcribed as /səˈpiːnə/, without such a sequence, so it doesn't count.