Append one mids object to another. Both objects are expected to have the same variables.
mids.append(x, y)
mids object a new mids object that contains all of `x` and the additional data in `y`
mids object provides both data and specification of imputation procedure
mids object only data information will be retained in the combined object
Only the data specific aspects are copied (i.e. $data, $imp, $where, $nmis), all other information in `y` is discarded. Therefore, only the imputation model of `x` is kept and `y` must not contain missing data in variables that did not have missing data in `x` (but the reverse is allowed).