#Substitute y when element of x is missing
#also return an attribute "substi.source"=vector of var names and NAs
substi <- function(x,y,pr=TRUE)		{

if(length(x)!=length(y))stop("lengths of x and y are different")
nf <- is.category(x)+is.category(y)
if(nf==1)stop("both x and y must be category variables if either is")

isna <- is.na(x)
vnames <- sys.call()[c(2,3)]
if(pr)	{
cat("Variables:",vnames,"\n")
cat("Used first  variable:",sum(!is.na(x)),"\n")
cat("Used second variable:",sum(is.na(x) & !is.na(y)),"\n") }
if(nf)					{
	levs <- unique(c(levels(x),levels(y)))
	x <- as.character(x)
	y <- as.character(y)
	x[isna] <- y[isna]
	x <- factor(x,levs)
	y <- factor(y,levs)		}
else x[isna] <- y[isna]
ss <- ifelse(isna & is.na(y),NA,ifelse(isna,2,1))
attr(ss,"names") <- NULL
ss <- factor(ss,labels=vnames)
if(pr)cat("Obs:",sum(!is.na(x))," Obs missing:",sum(is.na(x)),"\n")
attr(x,"substi.source") <- ss
attr(x,'class') <- c("substi",attr(x,'class'))
x
				}

substi.source <- function(x) attr(x,"substi.source")

"[.substi" <- function(x, ...) {
  ss <- attr(x,"substi.source")
  ats <- attributes(x)
  ats$dimnames <- ats$dim <- ats$names <- ats$substi.source <-
    attr(x,'class') <- NULL
  x <- (x)[...]
  attributes(x) <- ats
  attr(x,"substi.source") <- ss[...]
  x
}

print.substi <- function(x, ...) {

        i <- oldUnclass(attr(x, "substi.source"))
        if(!length(i)) {
                print.default(x)
                return(invisible())
        }
        if(is.factor(x))
                w <- as.character(x)
        else w <- format(x)
        names(w) <- names(x)
        w[i==2] <- paste(w[i==2], "*", sep = "")
        attr(w, "label") <- attr(w, "substi.source") <- attr(w, "class") <- NULL
        print.default(w, quote = FALSE)
        invisible()
}

as.data.frame.substi <- function(x, row.names = NULL, optional = FALSE, ...)
{
        nrows <- length(x)
        if(!length(row.names)) {
# the next line is not needed for the 1993 version of data.class and is
# included for compatibility with 1992 version
                if(length(row.names <- names(x)) == nrows && !any(duplicated(
                        row.names))) {
                }
                else if(optional)
                        row.names <- character(nrows)
                else row.names <- as.character(1:nrows)
        }
        value <- list(x)
        if(!optional)
                names(value) <- deparse(substitute(x))[[1]]
        structure(value, row.names=row.names, class='data.frame')
}