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