(** [gemv ?m ?n ?beta ?ofsy ?incy ?y ?trans ?alpha ?ar ?ac a ?ofsx ?incx x]
- see BLAS documentation!
+ see BLAS documentation! BEWARE that, when [n=0], this function
+ has no effect while it should compute [y ← beta * y].
@return vector [y], which is overwritten.
@param m default = number of available rows in matrix [a]
@param n default = available columns in matrix [a]