prov:value
| - R.sub.4, independently of one another, are alkenyl, aryl or cyanoalkyl; and (c) X.sub.2 --R--X.sub.2, where R is a divalent organic radical having up to 20 carbon atoms, and each X.sub.2, independently of the others, is --NHCO--, --CONH--, --NHCONH--,
|