title: How to drop some interaction terms from R formula
body: <p>I would like to drop some interaction terms from an R formula.  My situation is that I have one factor variable with lots of levels (call this A, and it takes values from 1-50), and another continuous variable that I would like to interact it with (call this B).</p>
  
  <pre><code>A*B
  </code></pre>
  
  <p>creates terms A1:B, A2:B, A3:B,...  I want a simple way to get rid of the first A1:B term.</p>
  
  <p>Note: I saw some previous answers for the lm case that called update and then dropped some terms.  This will not work for me as I am trying to estimate a multinomial logit model with the mlogit package, and I cannot make the first estimation without dropping some interactions.</p>
  
  <p>Edit: Although I am not attempting to use lm, if I could the following to occur, then I think it would solve my problem.</p>
  
  <pre><code>dd&lt;-data.frame(A=sample(letters[1:10], 100, replace=T),B = runif(100),z=rexp(100))
  
  #need to drop B term below
  reg1 &lt;- lm(z~A*B, dd)
  
  #or need to drop Aa:B term here
  reg2 &lt;- lm(z~A*B - B, dd)
  
  #but this doesn't work (I realize why, but this is an
  #example of what I would like to have happen)
  reg3 &lt;- lm(z~A*B - B - Aa:B, dd)
  </code></pre>