“‘When shows close, the book remains, the score remains, but there’s really no notation for choreography,’ said Nikki Feirt Atkins, the producing artistic director of Dance Machine, which is committed to passing on musical-theater dances from one performer to the next. She gestured toward Ms. de Lappe, who was showing some young men how to rein in their imaginary horses. ‘This is the living mission statement of the company.'”