My seven-year-old son suddenly had the itch to learn piano. I was wholeheartedly supportive.   The next day, my husband found an old classmate of his to be our son's piano teacher.   Then one day, when I returned home to retrieve a forgotten document, I heard my son's affectionate voice from inside.   "Mommy, does my playing sound good?"   I froze in place, and the next second, I heard my husband's voice.   "Once that woman is dead, we can be a real family, the three of us."