double(length = 0) as.double(x) is.double(x)
double creates a double precision vector of the specified
length. The elements of the vector are all equal to 0.
as.double attempts to coerce its argument to be of double
type.
is.double returns TRUE or FALSE depending on
whether its argument is of double type or not.
integer.is.double(1) all(double(3) == 0)