-- arithmetic
unknown + 1: unknown cannot be used in arithmetic
nothing + 1: arithmetic operator '+' expected number but got nothing
-- ordering
unknown > 5: unknown and nothing support only = and !=
-- a condition
if unknown: unknown cannot be used as a condition
if nothing: took the no branch
-- equality, which answers
unknown = 5        false
unknown = ""       false
unknown = unknown  true
nothing = 0        false
nothing = nothing  true
unknown = nothing  false
