impute <- function(x, what=c("median", "mean")){    what <- match.arg(what)        if(what == "median"){        retval <-            apply(x, 2,                  function(z) {z[is.na(z)] <- median(z, na.rm=TRUE); z})    }    else if(what == "mean"){        retval <-            apply(x, 2,                  function(z) {z[is.na(z)] <- mean(z, na.rm=TRUE); z})    }    else{        stop("`what' invalid")    }    retval}