Data Format Description Language (DFDL) v1.0 Specification
OGF Proposed Recommendation GFD-P-R.174, January 31, 2011
Some members may sometimes have the value no value, and it is said that such a member has no value. This value is distinct from all other values. In particular it is distinct from the empty string, the empty set, and the empty list, each of which simply has no members, and the special value nil.
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