function _nthchild($f, $c)
	{
		// $f is formula e.g. 2N+1 split into a preg_match array
		// $c is the comparator value e.g row or column number
		$c += 1;
		$select = false;

		$f_count = count($f);
		if ($f[0] === 'ODD') {
			$a = 2;
			$b = 1;
		} elseif ($f[0] === 'EVEN') {
			$a = 2;
			$b = 0;
		} elseif ($f_count === 2) {
			$a = 0;
			$b = $f[1] + 0;
		} // e.g. (+6)
		elseif ($f_count === 3) {  // e.g. (2N)
			if ($f[2] == '') {
				$a = 1;
			} elseif ($f[2] == '-') {
				$a = -1;
			} else {
				$a = $f[2] + 0;
			}
			$b = 0;
		} elseif ($f_count === 4) {  // e.g. (2N+6)
			if ($f[2] == '') {
				$a = 1;
			} elseif ($f[2] == '-') {
				$a = -1;
			} else {
				$a = $f[2] + 0;
			}
			$b = $f[3] + 0;
		} else {
			return false;
		}
		if ($a > 0) {
			if (((($c % $a) - $b) % $a) === 0 && $c >= $b) {
				$select = true;
			}
		} elseif ($a == 0) {
			if ($c == $b) {
				$select = true;
			}
		} else {  // if ($a<0)
			if (((($c % $a) - $b) % $a) === 0 && $c <= $b) {
				$select = true;
			}
		}
		return $select;
	}