1 print(4 and 5)          -- 5
 2 print(nil and 13)       -- nil
 3 print(false and 12)     -- false
 4 print(4 or 5)           -- 4
 5 print(false or 5)       -- 5
 6 
 7 print((4 and 5) or 6)      -- 5
 8 print((false and 5) or 6)  -- 6
 9 print((4 and false) or 6)  -- 6
10 
11 print(not nil)      -- ture
12 print(not false)    -- true
13 print(not 0)        -- false
14 print(not not nil)  -- false
15 
16 x = 10
17 v = 20
18 y = x or v
19 print(y)  -- 10