  The paper demonstrates the non-closure of the family of unambiguous linear
languages (that is, those defined by unambiguous linear context-free grammars)
under complementation. To be precise, a particular unambiguous linear grammar
is presented, and it is proved that the complement of this language is not
defined by any context-free grammar. This also constitutes an alternative proof
for the result of Hibbard and Ullian ("The independence of inherent ambiguity
from complementedness among context-free languages", J.ACM, 1966) on the
non-closure of the unambiguous languages under complementation.
