Finds first character in string that is in findchars, skipping all occurances of characters in skipchars. Search begins at pos. If no such characters are found, returns -1.
If skipchars is empty, skip anything not in findchars If skipchars is #f, skip nothing If findchars is empty, the first character not in skipchars is matched It is an error if findchars is not a string. It is an error if findchars is empty and skipchars is not a non-empty string.
(define (find-first-char string skipchars findchars #!optional (pos 0))
  ;; Find the first occurance of a character in a string
  (let ((skiplist (if (string? skipchars)
		      (string->list skipchars)
		      '()))
	(findlist (string->list findchars)))
    (if (and (null? skiplist) (null? findlist))
	;; this is an error
	-2
	(if (or (>= pos (string-length string)) (< pos 0))
	    -1
	    (let ((ch (string-ref string pos)))
	      (if (null? skiplist) 
		  ;; try to find first
		  (if (member ch findlist)
		      pos
		      (if (string? skipchars)
			  (find-first-char string 
					   skipchars findchars (+ 1 pos))
			  -1))
		  ;; try to skip first
		  (if (member ch skiplist)
		      (find-first-char string skipchars findchars (+ 1 pos))
		      (if (or (member ch findlist) (null? findlist))
			  pos
			  -1))))))))