title: rm(list=ls()) doesn't completely clear the workspace
body: <p>This is a very minor issue, but I would like to understand exactly what is going on here.</p>
  
  <p>Say I do the following:</p>
  
  <pre><code>library(RMySQL)
  con &lt;- dbConnect(MySQL(), host="some.server.us-east-1.rds.amazonaws.com",user="aUser", password="password", dbname="mydb")
  
  values1 &lt;- dbGetQuery(con,"select x,y from table1")
  attach(values1)
  </code></pre>
  
  <p>At this point, I can do</p>
  
  <pre><code>rm(list=ls())
  
  values2 &lt;- dbGetQuery("select x,y from table1")
  attach(values2)
  </code></pre>
  
  <p>but the attach gives me a warning about masking an x and y. I thought I had already clobbered those. What is going on? How do I completely clear a workspace?</p>