A Linguistics Workbook, 4th Edition

In other words, phonemes that possess the feature [+syllabic] are assigned the
feature [+long] whenever they appear before phonemes having the features
[+consonantal] and [+voiced].
The answers to exercises 3.3-3.6 require statements similar to rule 3. When only
a single phoneme appears in one part of the rule, however, you are usually not
required to give its unique distinctive feature specification, although this would be
required in a more technical statement of the rule.