system/Pager/PagerRenderer.php
public function getPrevious()
	{
		if (! $this->hasPrevious())
		{
			return null;
		}

		$uri = clone $this->uri;

		if ($this->segment === 0)
		{
			$uri->addQuery('page', $this->first - 1);
		}
		else
		{
			$uri->setSegment($this->segment, $this->first - 1);
		}

		return (string) $uri;
	}
public function getPrevious()
	{
		if (! $this->hasPrevious())
		{
			return null;
		}

		$uri = clone $this->uri;

		if ($this->segment === 0)
		{
			$uri->addQuery('page', $this->first - 1);
		}
		else
		{
			$uri->setSegment($this->segment, $this->first - 1);
		}

		return (string) $uri;
	}
Returns a URL to the "previous" page. The previous page is NOT the
page before the current page, but is the page just before the
"first" page.

You MUST call hasPrevious() first, or this value may be invalid.

@return string|null