(,moment-pair? . "pair of moment objects")
(,number-list? . "number list")
(,number-or-grob? . "number or grob")
+ (,number-or-markup? . "number or markup")
(,number-or-pair? . "number or pair")
(,number-or-string? . "number or string")
(,number-pair? . "pair of numbers")