title: How to include all possible two-way interaction terms in a linear model in R?
body: <p>Is there an easy way to include all possible two-way interactions in a model in R?</p>
  
  <p>Given this model:</p>
  
  <pre><code>lm(a~b+c+d)
  </code></pre>
  
  <p>What syntax would be used so that the model would include b, c, d, bc, bd, and cd as explanatory variables, were bc is the interaction term of main effects b and c. </p>