def provide(name)
      return nil unless name.respond_to?(:to_sym)
      name = name.to_sym
      fields[name] || fields[aliases[name]]
    end
def provide(name)
      return nil unless name.respond_to?(:to_sym)
      name = name.to_sym
      fields[name] || fields[aliases[name]]
    end
Returns the field value referenced by the passed-in name.
 For example, this will return the 'title' value for 'booktitle' if a
 corresponding alias is defined.