def [](code)
      subfield = self.find {|s| s.code == code}
      return subfield.value if subfield
      return
    end
def [](code)
      subfield = self.find {|s| s.code == code}
      return subfield.value if subfield
      return
    end
def each_by_code(filter)
  @subfields.each_by_code(filter)
end
 You can lookup subfields with this shorthand. Note it
 will return a string and not a MARC::Subfield object.
   subfield = field['a']