representing meta-values

5.10: adda/enums/the power of zero:
. the system should be interested in
reserving some values for representing
the state of being undefined or out-of-range .
. this though, can be done by system tagging;
ie, every type could inherit from system
to get a bit that indicates whether the var
is in a well-defined state or not .
. generally,
there is no reservable value available:
. most enum'value sequences start with zero
because most value ranges have a value
that is analogous to zero:
you first ask if there is any value,
if not then zero,
else since there is a value,
which one is it ?
eg, {off:0, red:1, yellow:2, green:3};
"(off) is the absence of a color,
so "(off) would be represented by zero;
[5.31: likewise,
some enums have a special purpose for the last value too;
so that can't be reserved either .]