SNESSetUp() and matrix-free options

[Jed Brown]

Barry Smith wrote:
> 
> On May 7, 2009, at 3:00 PM, Lisandro Dalcin wrote:

>> 2) What should happen if a user pass BOTH options -snes_mf and
>> -snes_mf_operators? Error? One of them should overrides the other?
> 
>    An error.

The tricky part here is, suppose the user wants to use one of these by
default, so they put it in a .petscrc or -options_file.  Now they will
get an error if they try to change it on the command line.  Is there a
reasonable way to use whichever option comes last?  Or just have
-snes_mf -snes_mf_type {...} so that the options system enforces that
there is only one way to do it.  -snes_mf_operator -pc_type none is
equivalent anyway.

Jed

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