(#%app expr ...)
Note that a use of _ that is nested within one of the given exprs does not count. Additionally, using _ after a keyword is not treated specially and does not trigger creation of a keyword argument. Rest arguments are not supported.
> (+ 1 (+ _ 2))
+: contract violation
argument position: 2nd
> (define (add/kw a #:to b) (+ a b)) > (define add2 (add/kw 2 #:to _)) > (add2 10)
> (add2 #:to 10)
application: procedure does not accept keyword arguments