title: bartlett.test fail with formula col1~col2+col3
body: <p>I wish to perform a <code>bartlett.test</code> with multiple independent grouping variables. However, an error is generated.</p>
  
  <p>Here is some data:</p>
  
  <pre><code>d=read.table(text='
  1 w e
  2 w e
  3 w r
  3 e r
  4 e r
  5 e e
  4 w r
  6 e e')
  </code></pre>
  
  <p>When I test the homogeneity of variance with one grouping variable only using formula V1 ~ V2, everything is perfect:</p>
  
  <pre><code>bartlett.test(V1 ~ V2, data = d)
  
      Bartlett test of homogeneity of variances
  
  data:  V1 by V2
  Bartlett's K-squared = 0, df = 1, p-value = 1
  </code></pre>
  
  <p>But when trying multiple independent variables, results in an error:</p>
  
  <pre><code>bartlett.test(V1 ~ V2 + V3, data = d)
  Error in bartlett.test.formula(V1 ~ V2 + V3, data = d) : 
    'formula' should be of the form response ~ group
  </code></pre>
  
  <p>And yet I want to test homogeneity of variance with multi-column, that is, test V1 ~ V2 + V3, so any assistance is appreciated.</p>