```ruby
#!/usr/bin/env ruby
# encoding: UTF-8

require 'rubygems'
require 'bundler/setup'

require_relative 'coordinate'

# Class to represent a detour, which holds a start and end Coordinate.
class Detour

  attr_accessor :start, :terminus

  def initialize(start, terminus)
    @start, @terminus = start, terminus
  end

  def valid?
    @start.nil? ? @terminus.nil? : !!@terminus
  end

  def invalid?
    not valid?
  end

  def no_distance?
    @start == nil || @terminus == nil || @start == @terminus
  end
end
```

Use ! instead of not
user
```ruby
#!/usr/bin/env ruby
# encoding: UTF-8

require 'rubygems'
require 'bundler/setup'

require_relative 'coordinate'

# Class to represent a detour, which holds a start and end Coordinate.
class Detour

  attr_accessor :start, :terminus

  def initialize(start, terminus)
    @start, @terminus = start, terminus
  end

  def valid?
    @start.nil? ? @terminus.nil? : !!@terminus
  end

  def invalid?
    !valid?
  end

  def no_distance?
    @start == nil || @terminus == nil || @start == @terminus
  end
end
```
assistant