code: def _string_in_table(self, candidate: str) -> List[str]:
          
          candidate_column_names: List[str] = []
          # First check if the entire candidate occurs as a cell.
          if candidate in self._string_column_mapping:
              candidate_column_names = self._string_column_mapping[candidate]
          # If not, check if it is a substring pf any cell value.
          if not candidate_column_names:
              for cell_value, column_names in self._string_column_mapping.items():
                  if candidate in cell_value:
                      candidate_column_names.extend(column_names)
          candidate_column_names = list(set(candidate_column_names))
          return candidate_column_names
docs: Checks if the string occurs in the table, and if it does, returns the names of the columns
          under which it occurs. If it does not, returns an empty list.