docs: def get_rule(self, field_id):
          """
          Returns the rule for the field identified by the id.
  
          If it is set as not being compulsory, the rule will be adapted to
          accept string composed only of white characters.
  
          :param field_id: unique id in the system for the field
          :return: the rule of a field
          """
  
          if field_id in self._fields:
              # Field already exists
              field = self._fields[field_id]
          else:
              # Field does not exist
              # It is created
              field = self._create_field(field_id)
  
              # Field is saved
              self._fields[field_id] = field
  
          return field