v input view w argument view
Unionmap returns a list of all the rows in w which are not in v, i.e. all the rows in w which have to be appended to v to produce the set union of v and w.
Example:set v [view A def {4 2 6}]
set w [view A def {1 2 3 4 5}]
view $v unionmap $w
0 2 4