def add(key, value)
    if @env.has_key?(key)
      @env[key] += value
    else
      set(key,value)
    end
  end
def add(key, value)
    if @env.has_key?(key)
      @env[key] += value
    else
      set(key,value)
    end
  end
Adds a value for key

 @param [String] key     Key that should be used for operation
 @param [String] value   Value that should be used for operation