If one wishes to describe the form-meaning relationship between 'sing' and 'song' as pertinent to the language (they certainly exibit partial phonetic-semantic resemblance), the structurally significant feature is the replacement of the vowel, not the addition of zero [derivative suffix].
- Nida (1948), a pag.427-428

[...] we treat noun plurals in English such as 'men', 'feet', 'mice', 'teeth' as occurring with 'replacives' (i.e. replacements which are morphemic).
- Nida (1948), a pag.429