code: func (f *FlagSet) Var(value Value, name string, usage string) {
  	f.VarP(value, name, "", usage)
  }
docs: // Var defines a flag with the specified name and usage string. The type and
  // value of the flag are represented by the first argument, of type Value, which
  // typically holds a user-defined implementation of Value. For instance, the
  // caller could create a flag that turns a comma-separated string into a slice
  // of strings by giving the slice the methods of Value; in particular, Set would
  // decompose the comma-separated string into the slice.