Consider the n-ary function 
.
The function g is exact if:
being closed over 
.
If g is exact then
.
The functions |x|, x, -x, 
, 
,
, and 
are all exact.
Allow 
to be an exact implementation of one
of the preceding exact functions. An implementation
of 
or 
may simply invoke .
| Jeff Tupper | March 1996 |