title: Removing duplicate combinations (irrespective of order)
body: <p>I have a data frame of integers that is a subset of all of the n choose 3 combinations of 1...n.
  E.g., for n=5, it is something like:</p>
  
  <pre><code>      [,1] [,2] [,3]
   [1,]    1    2    3
   [2,]    1    2    4
   [3,]    1    2    5
   [4,]    1    3    4
   [5,]    1    3    5
   [6,]    1    4    5
   [7,]    2    1    3
   [8,]    2    1    4
   [9,]    2    1    5
  [10,]    2    3    4
  [11,]    2    3    5
  [12,]    2    4    5
  [13,]    3    1    2
  [14,]    3    1    4
  [15,]    3    1    5
  [16,]    3    2    4
  [17,]    3    2    5
  [18,]    3    4    5
  [19,]    4    1    2
  [20,]    4    1    3
  [21,]    4    1    5
  [22,]    4    2    3
  [23,]    4    2    5
  [24,]    4    3    5
  [25,]    5    1    2
  [26,]    5    1    3
  [27,]    5    1    4
  [28,]    5    2    3
  [29,]    5    2    4
  [30,]    5    3    4
  </code></pre>
  
  <p>What I'd like to do is remove any rows with duplicate combinations, irrespective of ordering. E.g., <code>[1,]    1    2    3</code> is the same as <code>[1,]    2    1    3</code> is the same as <code>[1,]    3    1    2</code>.</p>
  
  <p><code>unique</code>, <code>duplicated</code>, &amp;c. don't seem to take this into account. Also, I am working with quite a large amount of data (n is ~750), so it ought to be a pretty fast operation. Are there any base functions or packages that can do this?</p>