def []=(key, val)
      key = key.to_sym
      raise EnvironmentError, "#{key} already binded" if @map.has_key?(key)
      @map[key] = val
    end
def []=(key, val)
      key = key.to_sym
      raise EnvironmentError, "#{key} already binded" if @map.has_key?(key)
      @map[key] = val
    end
Store the key/value pair.
 It is only possible to store a value for a key once, otherwise a error will be raised.