Emil, see my email just sent. We need to rename this function (and its 
Jacobian partner).

> On Oct 20, 2017, at 11:20 AM, Emil Constantinescu <[email protected]> 
> wrote:
> 
> 
> 
> Matt, that depends, if TS method is imex, then it computes just F, not F-G so 
> your argument is not correct. If the method can do only implicit it computes 
> F and subtracts G *if defined*. If the TS method can only do explicit and you 
> define F then it fails.
> 
> Again, this has to do with the TS methods and PETSc doing the work for you of 
> packing the functions in different ways.
> 
> Emil
> 
>>   Matt
>>    Now internally, because different solvers have different needs the
>>    IFunction ... presented to the TS solver may look differently. This
>>    is a design choice - if you are not a TS developer it should not
>>    affect you.
>>    This is a design decision: if implemented at this level, we avoid
>>    having every TS method be aware of the upper level functions.
>>    Emil
>> -- 
>> What most experimenters take for granted before they begin their experiments 
>> is infinitely more interesting than any results to which their experiments 
>> lead.
>> -- Norbert Wiener
>> https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>

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