A function which is not defined for all arguments of its input
type. E.g.
f(x) = 1/x if x /= 0.
f : D -@# C
may be represented as a total function
ft : D' -@# lift(C)
where D' is a superset of D and
ft x = f x if x in D
ft x = bottom otherwise
denotes "undefined".
(1995-02-03)