Zelig4: Everyone's Statistical Software

#' The \code{zelig2} function acts as a simple interface between a user's call
#' to the \code{zelig} function and the zelig functions subsequent call to the
#' pre-existing external model. The external model varies based on which model
#' is being called.
#'
#' @title Interface Between Zelig Models and External Functions
#' @note Writing \code{zelig2} functions is required of Zelig developers. In
#'   particular, \code{zelig2} functions act as an interface between external
#'   models (models not included in the Zelig package) and the \code{zelig}
#'   function which must use that model.
#'
#'   \code{zelig2} is not an actual function. Rather, 
#'
#' @name zelig2
#' @return
#'   The main purpose of the \code{zelig2} function is to return a list of
#'   key-value pairs, specifying how Zelig should interface with the external
#'   method. This list has the following format:
#'
#'   \item{\code{.function}}{specifies the name of the external method to be
#'     called by \code{zelig} function. Subsequent parameters, are called and
#'     evaluated as a function call to the function of the named string.}
#'   \item{\code{.hook}}{specifies the name of a hook function as a string. The
#'     hook function is only evaluated on zelig object once the external method
#'     fits the statistical model}
#'   \item{...}{any parameters aside from \code{.function} and \code{.hook} is 
#'     as part of the function call to the external model}
#'
#' @examples
#'  zelig2some.model <- function (formula, weights, verbose, ..., data) {
#'    list(
#'         .function = 'some.other.method',
#'         .hook = NULL,
#'         formula = formula,
#'         weights = 2 * weights,
#'         data = data
#'         )
#'  }
#'
#' ## This \code{zelig2} function equates the following function call:
#' ##  zelig(formula, weights = weights, verbose = TRUE, data = data, model="some.model")
#' ##
#' ## with:
#' ##  some.other.method(formula = formula, weights = 2 * weights, data=data)
#'
#' ## Note that the 'verbose' parameter is ignored, since the 
#' ## 'zelig2some.model' does not include the 'verbose' parameter in its return
#' ## value.
NULL