```ruby
class Pawn < Piece 
end
```

Add Pawn possible moves and spec
user
```ruby
class Pawn < Piece
  attr_reader :initial_position

  def initialize(color, position)
    super(color, position)
    @initial_position = position
  end

  def possible_moves
    moves = Set.new
    file = Piece::HOR_POS.index(self.position[0]) + 1
    rank = self.position[1].to_i

    self.color = :white ? direction = 1 : direction = -1

    moves.add("#{Piece::HOR_POS[file]}#{rank + (direction * 2)}") unless self.position != self.initial_position
    moves.add("#{Piece::HOR_POS[file]}#{rank + direction}")
    moves.add("#{Piece::HOR_POS[file - 1]}#{rank + direction}") unless invalid_file?(file - 1)
    moves.add("#{Piece::HOR_POS[file + 1]}#{rank + direction}")

    moves
  end

  def invalid_file?(file)
    file < 0 || !(1..9).include?(file)
  end
end
```
assistant