code: private void checkType(BtrpOperand e) {
          if (!(e instanceof BtrpNumber)) {
              throw new UnsupportedOperationException(e + " must be a '" + prettyType() + "' instead of a '" + e.prettyType() + "'");
          }
      }
docs: Check the operand is a number. If not, throws a {@link UnsupportedOperationException} exception.
  
  @param e the operand to check