code: def changeDefaultMessageConst(self, header_segment, segment_len, separator_count):
          
          self.header_segment = header_segment
          self.segment_len = segment_len
          self.separator_count = separator_count
docs: Provide a way to change default HL7 parameter if you want to subclass
          :param header_segment: Expected segment, for HL7, it's MSH
          :param segment_len: Expected length of segment, for HL7, it's 3
          :param separator_count: Expected count of separators, for HL7, it's 5
          :return: None