class Environment
  attr_reader :parent, :defaults

  def initialize(parent=nil, defaults={})
    @parent = parent
    @defaults = defaults
  end

  def define(symbol, value)
    defaults[symbol] = value
  end

  def defined?(symbol)
    defaults.has_key?(symbol) || (parent && parent.defined?(symbol))
  end

  def lookup(symbol)
    defaults[symbol] || (parent && parent.lookup(symbol)) ||
      raise("#{symbol} is undefined")
  end

  def set(symbol, value)
    if defaults.has_key?(symbol)
      defaults[symbol] = value
    elsif parent.nil?
      raise("#{symbol} was undefined")
    else
      parent.set(symbol, value)
    end
  end
end