| errors | Package errors implements functions to manipulate errors. |
| fmt | Package fmt implements formatted I/O with functions analogous to C's printf and scanf. |
| math/big | Package big implements arbitrary-precision arithmetic (big numbers). |
| math/rand | Package rand implements pseudo-random number generators suitable for tasks such as simulation, but it should not be used for security-sensitive work. |
| net | Package net provides a portable interface for network I/O, including TCP/IP, UDP, domain name resolution, and Unix domain sockets. |
| sync | Package sync provides basic synchronization primitives such as mutual exclusion locks. |