public function testCombinationsStatic()
 {
     // Size 0 combinations ---------------------------------------------------------------------
     $combinationList0 = Combination::get($this->_sourceDataSetKey, 0);
     $expectedList0 = [];
     $this->_assertCombination($combinationList0, $expectedList0, 0);
     // Size 1 combinations ---------------------------------------------------------------------
     $combinationList1 = Combination::get($this->_sourceDataSetKey, 1);
     $expectedList1 = [['a' => 5], ['b' => 6], ['c' => 8], ['d' => 10]];
     $this->_assertCombination($combinationList1, $expectedList1, 4, 1);
     // If this test pass, we don't need to check for other Index arrays as it proves that the
     // combination is still treated as a map and thus if the key arrays tests pass, so would the
     // indexed arrays
     $combinationIdxList1 = Combination::get($this->_sourceDataSetIdx, 1);
     $expectedIdxList1 = [[0 => 5], [1 => 6], [2 => 8], [3 => 10]];
     $this->_assertCombination($combinationIdxList1, $expectedIdxList1, 4, 1);
     // Size 3 combinations ---------------------------------------------------------------------
     $combinationList3 = Combination::get($this->_sourceDataSetKey, 3);
     $expectedList3 = [['a' => 5, 'b' => 6, 'c' => 8], ['a' => 5, 'b' => 6, 'd' => 10], ['a' => 5, 'c' => 8, 'd' => 10], ['b' => 6, 'c' => 8, 'd' => 10]];
     $this->_assertCombination($expectedList3, $combinationList3, 4, 3);
     // Size 4 combinations ---------------------------------------------------------------------
     $combinationList4 = Combination::get($this->_sourceDataSetKey, 4);
     $expectedList4 = [['a' => 5, 'b' => 6, 'c' => 8, 'd' => 10]];
     $this->_assertCombination($expectedList4, $combinationList4, 1, 4);
     // Size 3 combinations ---------------------------------------------------------------------
     $combinationList = Combination::get($this->_sourceDataSetKey);
     $expectedList = [['a' => 5], ['b' => 6], ['c' => 8], ['d' => 10], ['a' => 5, 'b' => 6], ['a' => 5, 'c' => 8], ['a' => 5, 'd' => 10], ['b' => 6, 'c' => 8], ['b' => 6, 'd' => 10], ['c' => 8, 'd' => 10], ['a' => 5, 'b' => 6, 'c' => 8], ['a' => 5, 'b' => 6, 'd' => 10], ['a' => 5, 'c' => 8, 'd' => 10], ['b' => 6, 'c' => 8, 'd' => 10], ['a' => 5, 'b' => 6, 'c' => 8, 'd' => 10]];
     $this->_assertCombination($expectedList, $combinationList, 15);
 }