text: const hexToBinary = (s) => {
      let ret = '';
      const lookupTable = {
          '0': '0000', '1': '0001', '2': '0010', '3': '0011', '4': '0100',
          '5': '0101', '6': '0110', '7': '0111', '8': '1000', '9': '1001',
          'a': '1010', 'b': '1011', 'c': '1100', 'd': '1101',
          'e': '1110', 'f': '1111'
      };
      for (let i = 0; i < s.length; i = i + 1) {
          if (lookupTable[s[i]]) {
              ret += lookupTable[s[i]];
          } else {
              return null;
          }
      }
      return ret;
  };
  
  export {hexToBinary};
  