w'>&lt;-</span> <span class='fu'>queue</span>()
 <span class='no'>q</span> <span class='kw'>&lt;-</span> <span class='fu'>insert</span>(<span class='no'>q</span>, <span class='fu'><a href='https://rdrr.io/r/base/list.html'>list</a></span>(<span class='fu'><a href='https://rdrr.io/r/base/environment.html'>environment</a></span>(), <span class='fu'><a href='https://rdrr.io/r/base/data.frame.html'>data.frame</a></span>(<span class='kw'>a</span><span class='kw'>=</span><span class='fl'>1</span>)))
 <span class=