of the affricates, because vowels tend to be short before most consonant clusters (excepting stops plus l or r, as in many European languages), but long before /ĉ/, /ĝ/, /c/, and /dz/, though again this varies by speaker, with some speakers pronouncing a short vowel before /ĝ/, /c/, /dz/ and a long vowel only before /ĉ/. Vowel quality has never been an issue for /a/, /i/ and /u/, but has been much discussed for /e/ and /o/. Zamenhof recommended pronouncing the vowels /e/ and /o/ as mid at all times. Kalocsay and Waringhien gave more complicated recommendations. For example, they recommended pronouncing stressed /e/, /o/ as short open-mid in closed syllables and long close-mid in open syllables. However, this is widely considered unduly elaborate, and Zamenhof's recommendation of using mid qualities is considered the norm. For many speakers, however, the pronunciation of /e/ and /o/ reflects the details of their native language. Epenthesis Zamenhof noted that epenthetic glides may be inserted between dissimilar vowels, especially after high vowels as in for ('my'), for ('honey') and for ('further'). This is quite common, and there is no possibility of confusion, because /ij/ and /uŭ/ do not occur in Esperanto (though more general epenthesis could cause confusion between and , as mentioned above). However, Zamenhof stated that in "severely regular" speech such epenthesis would not occur. Epenthetic glottal stops in vowel sequences such as ('boa') are non-phonemic detail, allowed for the comfort of the speaker. Glottal stop is especially common in sequences of identical vowels, such as ('hero'), and ('great-grandfather'). Other speakers, however, mark the hiatus by a change of intonation, such as by raising the pitch of the stressed vowel: . As in many languages, fricatives may become affricates after a nasal, via an epenthetic stop. Thus, the neologism ('sense', as in the five senses) may be pronounced the same as the fundamental word ('sense, meaning'), and the older term for the former, , may be preferable. Poetic elision Vowel elision is allowed with the grammatical suffix -o of singular nominative nouns, and the a of the article la, though this rarely occurs outside of poetry: ('from the heart'). Normally semivowels are restricted to offglides in diphthongs. However, poetic meter may force the reduction of unstressed and to semivowels before a stressed vowel: ; . Assimilation Zamenhof recognized place-assimilation of nasals before another consonant, such as n before a velar, as in