conditional dependencies & loading

Hi!

I'm working on a package that must convert data to and from JSON.  For this,
it can use either the rjson package, or preferably, the faster RJSONIO
package.

I have two related questions about this.

First, how can I specify that the package depends on *either* RJSONIO *or*
rjson?  (I.e. both are not required.)
  

Second, what's the best-practice R idiom for such conditional loading?  I.e.
I'm looking for R's equivalent of Python's

  

try:
        

...     import json
... except ImportError:
...     import simplejson as json

  

Thanks!

kynn

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