code: def findOrDie(s):
      
      aa = find(s)
      if aa:
          return aa
      else:
          print('Unknown amino acid or codon: %s' % s, file=sys.stderr)
          print('Valid arguments are: %s.' % list(CODONS.keys()),
                file=sys.stderr)
          sys.exit(1)
docs: Look up an amino acid.
  
      @param s: A C{str} amino acid specifier. This may be a full name,
          a 3-letter abbreviation or a 1-letter abbreviation. Case is ignored.
      @return: An C{AminoAcid} instance, if one can be found. Else exit.