func: function getFirstDocumentIdStartingAtValidFrom(uint _unixTimeFrom) public view
      returns (uint) 
      {
          for (uint i = 0; i < documentsCount; i++) {
             Document memory doc = documents[i];
             if (doc.validFrom>=_unixTimeFrom) {
                 return i;
             }
          }
          return 0;
      }