docs: public String getMessage() {
          switch ( this.errorCode ) {
              case ERROR_UNUSED_TOKEN :
                  return "Warning, the token " + this.token + " not used in the mapping.";
              case ERROR_UNDECLARED_TOKEN :
                  return "Warning, the token " + this.token + " not found in the expression. (May not be a problem).";
              case ERROR_INVALID_TOKEN :
                  return "Invalid token declaration at offset " + this.offset + ": " + this.token;
              case ERROR_UNMATCHED_BRACES :
                  return "Unexpected } found at offset " + this.offset;
              default :
                  return "Unkown error at offset: " + this.offset;
          }
      }