Nothing
### Copyright (C) 1998
### Berwin A. Turlach <bturlach@stats.adelaide.edu.au>
### Bill Venables <wvenable@stats.adelaide.edu.au>
### --> ../COPYRIGHT for more details
is.formula <- function(x) inherits(x, "formula")
merge.formula <- function(x, y, ...)
{
if(!is.formula(x) || length(x) != 3)
stop("First argument is invalid")
if(!is.formula(y)) stop("Second argument is invalid")
if(length(list(...))) warning("extraneous arguments discarded")
is.gEnv <- function(e) identical(e, .GlobalEnv)
str <- paste(c(deparse(x[[2]]), "~",
deparse(x[[3]]), "+",
deparse(y[[length(y)]])), collapse = "")
f <- as.formula(str)
## MM: try to keep environment (where reasonable)
ex <- environment(x)
ey <- environment(y)
if(!is.gEnv(ex)) {
environment(f) <- ex
if(!is.gEnv(ey) && !identical(ex,ey)) {
warning("`x' and `y' have different environments; x's is used")
}
} else if(!is.gEnv(ey))
environment(f) <- ey
f
}
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