If @var{data} is @code{#f} or @code{'()}, it is not included in the sum."
(cond ((or (not data) (null? data))
func)
- ((or (ly:unpure-pure-container func)
- (ly:unpure-pure-container data))
+ ((or (ly:unpure-pure-container? func)
+ (ly:unpure-pure-container? data))
(ly:make-unpure-pure-container
(lambda rest
(plus (apply ly:unpure-call func rest)