Scheme syntax-rules matching any number of pattern variable before a literal?

Question

I'm trying to write a macro that substitutes some literal in an expression with a value, such as (substitute 3 (+ 4 1 _ 1 5))

This is what I have so far,

(define-syntax substitute
  (syntax-rules (_)
    ((substitute val (_ e1 ...))
     (val e1 ...))
    ((substitute val (e1 _ e2 ...))
     (e1 val e2 ...))
    ((substitute val (e1 e2 _ e3 ...))
     (e1 e2 val e3 ...))))

How can I generalize this? Or really, how should I approach this?

Answer 1

Ok, I got a solution.

(define-syntax sub-apply
  (syntax-rules ()
    ((sub-apply val (op e ...))
     (apply op
            (sub-list val (e ...))))))

(define-syntax sub-list
  (syntax-rules (_)
    ((sub-list val (_)) '(val))
    ((sub-list val (e)) '(e))
    ((sub-list val (_ e ...))
     (cons val (sub-list val (e ...))))
    ((sub-list val (e1 e2 ...))
     (cons e1 (sub-list val (e2 ...))))))

edit.
This is even better! It works with special constructs.

;; usage:
;; (undswap 3 _) => 3
;; (undswap 3 (if _ (+ 3 _ ))) => 6
(define-syntax undswap
  (syntax-rules (_)
    ((undswap val (e ...))
     ((undswap val e) ...))
    ((undswap val _) val)
    ((undswap val e) e)))