fix 2.16 automatic reference counting
fix 2.17 optional chainning
fix 2.18 type casting
fix 2.19 Nested
fix 2.20 extensions
fix 2.21 protocols
fix 2.22 generices
fix 2.23 advaced operators
fix 3.1 about the language reference
fix 3.2 lexical structure
fix 3.3 types
fix 3.3 types(2)
fix 3.4 expressions
fix 3.5 declarations
fix 2.16 automatic reference counting
fix 2.17 optional chainning
fix 2.18 type casting
fix 2.19 Nested
fix 2.20 extensions
merge 2.21 protocols