code: public function setUpper($upper): self
      {
          if ( ! is_numeric($upper)) {
              throw new InvalidArgumentException(
                  __METHOD__ . "() expects parameter one, upper, to be a number"
              );    
          }
          
          $this->upper = $upper;
          
          return $this;
      }
docs: Sets the interval's upper value
  
  @param   int|float  $upper
  @return  self
  @throws  InvalidArgumentException  if $upper is not a number
  @since   0.1.0