title: Exclude one factor level interaction calculation in lme4
body: <p>I am making mixed models, using <code>lme4</code>, which included three factors (S,M,R) with 2,3 and 4 levels. But I am missing observations for S3:M2:R1, so I am not able to check for the 3-way interaction. </p>
  
  <p>Is there any way to tell <code>lme4</code> to ignore estimating a parameter for this level? </p>
  
  <p>Instead of it giving me the message: <code>Error in mer_finalize(ans) : Downdated X'X is not positive definite, 21.</code></p>