title: Construct an R package vignette that reads in an example .rda file?
body: <p>I have a similar to question to what was posted here, and tried to set up my folders in a similar manner: (<a href="https://stackoverflow.com/questions/11421559/writing-an-r-package-vignette-that-reads-in-an-example-file">Writing an R package vignette that reads in an example file?</a>)</p>
  
  <p>I'm writing a vignette for a package in R.</p>
  
  <p>I made a .Rnw file and put it into a subdirectory inst/doc inside my package pV. Inside the same subdirectory inst/doc, I put a folder example containing a .rda file called tree.rda.</p>
  
  <p>Before I can run any of the functions in the vignette, I have to read in the tree.rda file (this is lines 13-21):</p>
  
  <pre><code>```{r}
  library(pV)
  library(plyr)
  library(reshape2)
  library(ggplot2)
  library(stringr)
  library(igraph)
  system.file('tree.rda', 'example', package = 'pV')
  load("tree.rda")
  ```
  </code></pre>
  
  <p>but get the error:</p>
  
  <pre><code>* checking for file '/Users/MacOwner/Desktop/pV/DESCRIPTION' ... OK
  * preparing 'pV':
  * checking DESCRIPTION meta-information ... OK
  * installing the package to build vignettes
  * creating vignettes ... ERROR
  Quitting from lines 13-21 (pV.rmd) 
  Error: processing vignette 'pV.rmd' failed with diagnostics:
  cannot open the connection
  Execution halted
  Error: Command failed (1)
  </code></pre>
  
  <p>How can I successfully read in the .rda file so that my next command could be (and show the first lines of tree.rda file):</p>
  
  <pre><code>```{r}
  head(tree)
  ```
  </code></pre>
  
  <p>and so that I can use this tree object as input to additional functions that require it as an input in the later parts of the vignette? </p>