code: def get_smallest_compound_id(compounds_identifiers):
      
      return min((c for c in compounds_identifiers if c.startswith("C")),
                 key=lambda c: int(c[1:]))
docs: Return the smallest KEGG compound identifier from a list.
  
      KEGG identifiers may map to compounds, drugs or glycans prefixed
      respectively with "C", "D", and "G" followed by at least 5 digits. We
      choose the lowest KEGG identifier with the assumption that several
      identifiers are due to chirality and that the lower one represents the
      more common form.
  
      Parameters
      ----------
      compounds_identifiers : list
          A list of mixed KEGG identifiers.
  
      Returns
      -------
      str
          The KEGG compound identifier with the smallest number.
  
      Raises
      ------
      ValueError
          When compound_identifiers contains no KEGG compound identifiers.