Using 'not_function' it is possible to match functions other than
a specific set of ones. That will make it significantly easier to
allow certain functions to be used with certain columns.
The special handling of no arguments to `function` in conjunction
with an allowing filter has been removed. The same effect can now
be achieved, without special handling, using `not_function`, no
arguments and a blocking filter.
Implementation will follow in a subsequent commit.
3215458b30