# Functions
WithCancel create a new Group and an associated Context derived from ctx.
WithCancelForIndexGroup 创建ForIndexGroup带Context和Cancel.
WithContext create a Group.
WithContextForIndexGroup 创建ForIndexGroup带Group.
# Structs
ForIndexGroup 是一个引用for index执行多个携程的ErrGroup.
A Group is a collection of goroutines working on subtasks that are part of the same overall task.