code: def process_sub(ref, alt_str):
      
      if len(ref) == len(alt_str):
          if len(ref) == 1:
              return record.Substitution(record.SNV, alt_str)
          else:
              return record.Substitution(record.MNV, alt_str)
      elif len(ref) > len(alt_str):
          return process_sub_grow(ref, alt_str)
      else:  # len(ref) < len(alt_str):
          return process_sub_shrink(ref, alt_str)
docs: Process substitution