clap/examples/tutorial_derive/03_01_flag_bool.md

$ 03_01_flag_bool_derive --help
A simple to use, efficient, and full-featured Command Line Argument Parser

Usage: 03_01_flag_bool_derive[EXE] [OPTIONS]

Options:
  -v, --verbose  
  -h, --help     Print help information
  -V, --version  Print version information

$ 03_01_flag_bool_derive
verbose: false

$ 03_01_flag_bool_derive --verbose
verbose: true

$ 03_01_flag_bool_derive --verbose --verbose
? failed
error: The argument '--verbose' was provided more than once, but cannot be used multiple times

Usage: 03_01_flag_bool_derive[EXE] [OPTIONS]

For more information try '--help'