To do:

Look at fnDeriv:  producing a function.
 name current fnDeriv as something like evalDeriv. DONE (used codeDeriv as name)
 
Look into ways of getting variables. ..., env, data, etc.
NO CHANGES

Nice filter function to handle user input.  John will suggest names. DONE using dex().

Fail for two-sided formula.  DONE, but just a warning.

JN: I agree that it isn't worth doing at this level of effort. Maybe we
just note that using our function on a 2-sided formula has possibly undefined
consequences.??

Get R CMD check working. DONE
JN: Still getting same error on Linux Mint 18. ?? 
    I'll keep looking into this and see if I can find source of issue.

"* checking PDF version of manual ... WARNING
LaTeX errors when creating PDF version.
This typically indicates Rd problems.
* checking PDF version of manual without hyperrefs or index ... OK
* DONE
Status: 1 WARNING"

