`"array_or_vector"`

is the class union of
`c("array", "matrix", "vector")`

and exists for its use in
signatures of method definitions.

Using `"array_or_vector"`

instead of just `"vector"`

in a
signature makes an important difference: E.g., if we had
`setMethod(crossprod, c(x="mpfr", y="vector"), function(x,y) CPR(x,y))`

,
a call `crossprod(x, matrix(1:6, 2,3))`

would extend into a call
of `CPR(x, as(y, "vector"))`

such that `CPR()`

's second
argument would simply be a vector instead of the desired
*2 x 3* matrix.

A virtual Class: No objects may be created from it.

1 | ```
showClass("array_or_vector")
``` |

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