判断句子是否是全字母句。全字母句(希腊语:παγρμμα,pan 语法,”每个字母”)是一个使用字母表中每个字母，至少一次的句子。最著名的英语是 全字母句:

The quick brown fox jumps over the lazy dog.

字母表由 ASCII 字母a到z的全部组成，并且不区分大小写。输入不能包含非 ASCII 符号.

维基百科https://en.wikipedia.org/wiki/Pangram

/// Determine whether a sentence is a pangram.
pub fn is_pangram(sentence: &str) -> bool {
   unimplemented!("Is {} a pangram?", sentence);
}

# #![allow(unused_variables)]
#fn main() {
#[test]
fn empty_strings_are_not_pangrams() {
   let sentence = "";
   assert!(!is_pangram(&sentence));
}

#[test]
//#[ignore]
fn classic_pangram_is_a_pangram() {
   let sentence = "the quick brown fox jumps over the lazy dog";
   assert!(is_pangram(&sentence));
}

#[test]
//#[ignore]
fn pangrams_must_have_all_letters() {
   let sentence = "a quick movement of the enemy will jeopardize five gunboats";
   assert!(!is_pangram(&sentence));
}

#[test]
//#[ignore]
fn pangrams_must_have_all_letters_two() {
   let sentence = "the quick brown fish jumps over the lazy dog";
   assert!(!is_pangram(&sentence));
}

#[test]
//#[ignore]
fn pangrams_must_include_z() {
   let sentence = "the quick brown fox jumps over the lay dog";
   assert!(!is_pangram(&sentence));
}

#[test]
//#[ignore]
fn underscores_do_not_affect_pangrams() {
   let sentence = "the_quick_brown_fox_jumps_over_the_lazy_dog";
   assert!(is_pangram(&sentence));
}

#[test]
//#[ignore]
fn numbers_do_not_affect_pangrams() {
   let sentence = "the 1 quick brown fox jumps over the 2 lazy dogs";
   assert!(is_pangram(&sentence));
}

#[test]
//#[ignore]
fn numbers_can_not_replace_letters() {
   let sentence = "7h3 qu1ck brown fox jumps ov3r 7h3 lazy dog";
   assert!(!is_pangram(&sentence));
}

#[test]
//#[ignore]
fn capitals_and_punctuation_can_be_in_pangrams() {
   let sentence = "\"Five quacking Zephyrs jolt my wax bed.\"";
   assert!(is_pangram(&sentence));
}

#[test]
//#[ignore]
fn non_ascii_characters_can_be_in_pangrams() {
   let sentence = "Victor jagt zwölf Boxkämpfer quer über den großen Sylter Deich.";
   assert!(is_pangram(&sentence));
}

#}

# #![allow(unused_variables)]
#fn main() {
use std::collections::BTreeSet;
use std::iter::FromIterator;

pub fn is_pangram(sentence: &str) -> bool {
   sentence
       .to_lowercase()
       .chars()
       .filter(|c| c.is_alphabetic())
       .filter(|c| c.is_ascii())
       .collect::<BTreeSet<char>>() == english_letter_set()
}

fn english_letter_set() -> BTreeSet<char> {
   BTreeSet::from_iter(ENGLISH_ALPHABET.chars())
}

const ENGLISH_ALPHABET: &'static str = "abcdefghijklmnopqrstuvwxyz";

#}