title: linearHypothesis() to test coefficients of terms enclosed in I(X)
body: <p>I was about test the coefficients with lht command in the following model: </p>
  
  <pre><code>fashion.lm&lt;-lm(LOGS~D2+D3+D4+I(D1*LOGA)+I(D2*LOGA)+I(D3*LOGA)+I(D4*LOGA)+I(D1*LOGC)+I(D2*LOGC)+I(D3*LOGC)+I(D4*LOGC))
  </code></pre>
  
  <p>However, when I try to put <code>I(D1*LOGA)</code> into <code>lht()</code> function, it generates errors:</p>
  
  <pre><code>library(car)
  lht(fashion.lm,c("I(D1*LOGA)"))
  
  &gt; lht(fashion.lm,c("I(D1*LOGA)"))
  Error in constants(lhs, cnames_symb) : 
    The hypothesis "I(D1*LOGA)" is not well formed: contains bad coefficient/variable names.
  In addition: Warning message:
  In constants(lhs, cnames_symb) : NAs introduced by coercion
  </code></pre>
  
  <p>I was wondering how to properly do the test in the model? I know one (not so smart method) is to create a variable with the values equal to <code>D1*LOGA</code> before I run the regression. But is there a more convenient way or doing it? </p>