2 :D expanded when var set
5 :U expanded when var undef
8 :D skipped if var undef
10 :U skipped when var set
12 :? only lhs when value true
15 :? only rhs when value false
18 do not evaluate or expand :? if discarding
20 year=2016 month=04 day=01
22 Version=123.456.789 == 123456789
23 Literal=3.4.5 == 3004005
24 We have target specific vars