Possible fix for issue 261 ?

Hi

This is my very first use of gitHub so please forgive (but do correct) any protocol breaches I have blundered into.
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Update 13/8/2014
I've added another proposed change (as at 13/8/2014) as explained below, relating to reading the parsed roxygen comments in collate order sequence..

It is unlikely to be suitable verbatim, but it may save some time when a more experienced programmer looks into what it is trying to do.  I intended to make it a separate "pull" request to my initial issue 261 suggestion, but I am still rather inexperienced with git and github.
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I have had a go at correcting what I think is the source of the problem in issue 261 https://github.com/klutometis/roxygen/issues/261
namely that 
```top <- eval(help_call)```  
in roxygen2::get_rd raises an error condition if help cannot find the documentation requested (as well as returning a ``top''' which has length == 0.)

This is also my first attempt at using tryCatch and conditions, so I'm sure the code can be made more elegant.  It does seem to work on my toy example (in the sense that devtools::document() now completes running and generates the documentation I expect.)

I'm not sure what consequences this suggested change might have for elsewhere in the roxygen2 code though.  In particular if a user were to attempt to explicitly ```@inheritParams package::function``` from a package that was not being worked on by roxygen2 at that point in time, and generated a "Could not find ..." style error message, I'm unsure what would happen downstream in the roxygenising process.  (To be honest I'm not even sure if such an ```@inheritParams``` tag would end up in this segment of code.)

Hoping this is useful.

Geoff
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Hi

I have experienced a couple of related issues using the devtools / roxygen2 combination to develop and document a package.  Both issues relate to inheritance, and the file collation order for the package (which roxygen2 very nicely generates from the @include tags in the roxygen comments in my R code).  I am using devtools 1.5.0.99 and roxygen2 4.0.1

For details of the issues see :
  1) issue 261 , devtools::load_all not happy :
             https://github.com/klutometis/roxygen/issues/261
             https://groups.google.com/forum/#!topic/rdevtools/N3wy684ccnA
             https://groups.google.com/forum/#!topic/rdevtools/2TMDbDcPNA8
             and
  2) issue related to order of (inherited) parameters in documentation from roxygen2
             https://groups.google.com/forum/#!topic/rdevtools/KBd7X8UbqPg

These issues would be solved if :
  1) devtools::load_all updated the Collate field in the description file before attempting to load and evaluate the R code in the package
  and
  2) roxygen2::roxygenise read in and parsed the roxygen comment blocks from R files in the order of the Collate sequence instead of from R files in alphabetical filename sequence.

I have tried some 'proof of concept' tweaks to devtools::loadall and roxygen2::roxygenise to trial this, and they do solve my immediate problem.

My solutions are not as elegant and concise as the master devtools and roxygen code (which is awesome!), and have not been tested in other 'use cases', nor for efficiency, nor for other platforms (I am on Windows 7). But perhaps they will be useful small contribution as a rough starting point for other more experienced coders.

Regards

Geoff Lee