Four Tones - hear, hum and drill Mandarin tones

Hear the four Mandarin tones as hummed pitch contours, type any pinyin and see its spoken melody with tone changes worked out, drill the twenty tone pairs by ear - then get tone-numbered phrases for a real situation or have your own pinyin checked syllable by syllable. Built on @jackjin1997/language-learning (phrase sets) and @tatat/language-tutor (pinyin check).