as.bt <- function (df, value = NA) 
{
    if (class(df) != "data.frame") 
        stop("Can only convert data frames")
    facnames <- setdiff(names(df), value)
    toindx <- function(x) if (is.numeric(x)) 
        unique(x)
    else if (is.factor(x)) 
        levels(x)
    else as.character(unique(x))
    inds <- lapply(facnames, function(x) toindx(df[, x]))
    names(inds) <- facnames
    dims <- sapply(inds, length)
    dimtot <- prod(dims)
    vals <- sapply(1:dimtot, function(i) {
        okrows <- rep(TRUE, nrow(df))
        levswanted <- sapply(1:length(facnames), function(j) {
            nm <- facnames[[j]]
            nl <- ((i - 1)%/%cumprod(c(1, dims))[j])%%dims[j]
            lev <- inds[[nm]][nl + 1]
            okrows <<- okrows & df[, nm] == lev
            lev
        })
        levs <- paste(facnames, levswanted, sep = "=", collapse = " ")
        if (!any(okrows)) 
            warning("No entries for ", levs, call. = FALSE)
        if (length(which(okrows)) > 1) 
            warning("Multiple entries for", levs, call. = FALSE)
        x <- NA
        if (!is.na(value)) 
            if (any(okrows)) 
                x <- df[which(okrows)[1], value]
        if (!is.na(value) & !all(is.na(x)) & !is.numeric(x)) 
            x <- as.character(x)
        x
    })
    res <- vals
    class(inds) <- "btindexes"
    attr(res, "indexes") <- inds
    class(res) <- "bt"
    res
}