Tools for Assessing Estimability of Linear Predictions
c(person("Russell", "Lenth", role = c("aut", "cre", "cph"), 
    email = "russell-lenth@uiowa.edu"))
Provides tools for determining estimability of linear functions 
  of regression coefficients, and 'epredict' methods that handle 
  non-estimable cases correctly. Estimability theory is discussed in
  many linear-models textbooks including Chapter 3 of Monahan, JF (2008), 
  "A Primer on Linear Models", Chapman and Hall (ISBN 978-1-4200-6201-4).