setx#' This method is used internally
reduce(dataset, s, formula, data, avg = avg)Zelig object data, possibly split to deal with by
argument
list of variables and their tentative setx values
a simplified version of the Zelig object formula (typically with 1 on the lhs)
Zelig object data
function of data transformations
a list of all the model variables either at their central tendancy or
their setx value