('bank') and ('blood'), or before palatal , as in ('mommy') and ('sir'). However, he stated that "severely regular" speech would not have such variation from his ideal of 'one letter, one sound'. Nonetheless, although the desirability of such allophony may be debated, the question almost never arises as to whether the m in should remain bilabial or should assimilate to labiodental f (), because this assimilation is nearly universal in human language. Indeed, where the orthography allows (e.g. 'bonbon'), we see that assimilation can occur. In addition, speakers of many languages (including Zamenhof's, though not always English) have regressive voicing assimilation, when two obstruents (consonants that occur in voiced-voiceless pairs) occur next to each other. Zamenhof did not mention this directly, but did indicate it indirectly, in that he didn't create compound words with adjacent obstruents that have mixed voicing. For example, by the phonotactics of both of Zamenhof's mother tongues, Yiddish and (Belo)Russian, ('rose-colored', 'pink') would be pronounced the same as ('dew-colored'), and so the preferred form for the former is . Indeed, Kalocsay & Waringhien state that when voiced and voiceless consonants are adjacent, the assimilation of one of them is "inevitable". Thus one pronounces ('eighty') as , as if it were spelled ""; ('exist') as , as if it were spelled ""; ('for example') as , ('support') as , ('for a long time') as , ('ringing of a sword') as , etc.Miroslav Malovec, 1999, , §2.9. Such assimilation likewise occurs in words that maintain Latinate orthography, such as ('absolutely'), pronounced , and ('obtuse'), pronounced , despite the superficially contrastive sequences in the words ('apsis') and ('optics'). Instead, the debate centers on the non-Latinate orthographic sequence kz, frequently found in Latinate words like and above. It is sometimes claimed that kz is properly pronounced exactly as written, with mixed voicing, , despite the fact that assimilation to occurs in Russian, English (including the words 'example' and 'exist'), Polish (where it is even spelled ), French and many other languages. These two positions are called and in Esperanto. In practice, most Esperanto speakers assimilate kz to and pronounce nk as when speaking fluently. {|class=wikitable |+Voicing assimilation |- !Voiceless obstruent | p ||t ||c ||ĉ ||k ||f ||s ||ŝ ||ĥ |- !Pronunciation before any voiced obstruent but v| b ||d ||dz ||ĝ ||g ||v ||z ||ĵ || |- | colspan=10| |- !Voiced obstruent | b ||d ||dz ||ĝ ||g ||v