code: func (f *FlagSet) Float64Var(p *float64, name string, value float64, usage string) {
  	f.Var(newFloat64Value(value, p), name, usage)
  }
docs: // Float64Var defines a float64 flag with specified name, default value, and usage string.
  // The argument p points to a float64 variable in which to store the value of the flag.