| flag | Package flag implements command-line flag parsing. |
| fmt | Package fmt implements formatted I/O with functions analogous to C's printf and scanf. |
| go/ast | Package ast declares the types used to represent syntax trees for Go packages. |
| go/token | Package token defines constants representing the lexical tokens of the Go programming language and basic operations on tokens (printing, predicates). |
| go/types | Package types declares the data types and implements the algorithms for type-checking of Go packages. |
| reflect | Package reflect implements run-time reflection, allowing a program to manipulate objects with arbitrary types. |
| strings | Package strings implements simple functions to manipulate UTF-8 encoded strings. |
| unicode | Package unicode provides data and functions to test some properties of Unicode code points. |