it handles nodes, links, events(link_up, link_down)

it will _not_ work for arbitrary ns input files. The problem is that
an input file might say

set something [$ns some args]
$something do things

but my code doesn't return anything exciting in the first line.
I return Node, Link, Event, but if they are expecting an
Agent/whatever/whatever back, they are out of luck.
...and so there isn't anything for the second line to make do things
...and it'll break.