as.mitml.list <- function(x){
# adds a class attribute "mitml.list" to its argument

  if(!is.list(x)) stop("Argument must be a 'list'.")

  if(any(!sapply(x, is.data.frame))){
    x <- lapply(x, as.data.frame)
    cat("Note: List entries were converted to class 'data.frame'.\n")
  }

  class(x) <- c("mitml.list", class(x))
  return(x)
    
}