lib/srfi/srfi-1.scm: add append-map, filter, find.

lib/srfi/srfi-1.scm

@@ -0,0 +1,15 @@
+(define (find pred lst)
+  (let loop ((lst lst))
+    (if (null? lst) #f
+        (if (pred (car lst)) (car lst)
+            (loop (cdr lst))))))
+
+(define (filter pred lst)
+  (let loop ((lst lst))
+    (if (null? lst) '()
+        (if (pred (car lst))
+            (cons (car lst) (loop (cdr lst)))
+            (loop (cdr lst))))))
+
+(define (append-map f lst)
+  (apply append (map f lst)))