```
### Based on http://en.wikipedia.org/wiki/Phi_coefficient
phicoef <- function(x, y=NULL)
{
if (is.null(y)) {
if (!is.integer(x) || length(x) != 4L)
stop("when 'y' is not supplied, 'x' must be ",
"a 2x2 integer matrix or an integer vector of length 4")
a <- x[1L]
c <- x[2L]
b <- x[3L]
d <- x[4L]
} else {
if (!is.logical(x) || !is.logical(y) || length(x) != length(y))
stop("when 'y' is supplied, 'x' and 'y' must be ",
"2 logical vectors of the same length")
a <- sum(x & y)
b <- sum(x & !y)
c <- sum(!x & y)
d <- sum(!x & !y)
}
a <- as.double(a)
b <- as.double(b)
c <- as.double(c)
d <- as.double(d)
div <- sqrt((a + b) * (c + d) * (a + c) * (b + d))
(a * d - b * c) / div
}
```

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