* Example 1: 
* each basic event has a failure prob attached to it. 
ftree ft1
basic one prob(p1)
basic two prob(p2)
basic three prob(p3)
and 12 one two
and 23 two three
or top 12 23
end
bind
p1 0.1
p2 0.4
p3 0.7
end

* System unreliability is computed as 
var sysunrel sysprob(ft1)
expr sysunrel
end
