FALSE

UNDEF

Special TriStateBoolean object representing an
uninitialized value.  This is needed to support the
aggregate() operation.  Using the UNDEF value
is analogous to saying "no value".  When UNDEF is used
in a TriStateBoolean operation, it is essentially ignored,
the result of the operation is determined by the other operand.  If
both operands are UNDEF then UNDEF is
returned.