public function test_letter_frequency()
{
$words = new MapOfStrings(explode(' ', 'the cat in the hat'));
$freqs = $words->letter_frequency();
$this->assertInstanceOf('\\Haldayne\\Boost\\MapOfInts', $freqs);
$this->assertEquals(['a' => 2, 'c' => 1, 'e' => 2, 'h' => 3, 'i' => 1, 'n' => 1, 't' => 4], $freqs->toArray());
}
