[erlang-questions] Maps single value access not implemented

[Richard A. O'Keefe]

There are three different cases:

 - I want this field to be replaced.  It MUST exist.
 - I want this field to be added.  It must NOT exist.
 - I want this field to have this value and I DON'T CARE
   whether it existed before or not.

Both of the first two cases are important for detecting errors.

In the frames proposal, the *operation* was the same in all
cases, as I thought the checks should be delegated to the
Dialyzer.