code: def build_summary(val1=None, val2=None, val3=None):
          
          summary = []
          if val1 is not None:
              summary.append(val1)
          if val2 is not None:
              summary.append(val2)
          if val3 is not None:
              summary.append(val3)
          if not summary:
              return None
          return ' : '.join(summary)
docs: Constructs the summary given va1, va2, val3
  
          Args:
              val1:
              val2:
              val3:
  
          Returns: