Added intersection, member, and union functions.

These are neat to have.

PRIMITIVES.md

@@ -286,6 +286,8 @@ Functions here include:
   * Is the given thing a hash?
 * `inc`
   * Increment the given variable.
+* `intersection`
+  * Return those elements in common in the specified pair of lists.
 * `last`
   * Return the last element of the specified list.
 * `length`
@@ -304,6 +306,8 @@ Functions here include:
     * Return the results of applying the specified function to every pair of elements in the given list.
 * `max`
   * Return the maximum value in the specified list.
+* `member`
+  * Return true if the specified item is contained within the given list.
 * `min`
   * Return the maximum value in the specified list.
 * `nat`
@@ -362,6 +366,8 @@ Functions here include:
 * `translate`
   * Translate a string of characters, via a lookup table.
   * Used by `lower`, and `upper`.
+* `union`
+  * Return a list of all items in the specified two lists - without duplicates.
 * `upper`
   * Return an upper-case version of the specified string.
 * `upper-table`

stdlib/stdlib/lists.lisp

@@ -0,0 +1,39 @@
+;;; lists.lisp - Some list-utility functions
+
+;; These were adapted from Rob Pike's lisp
+;;
+;;    https://github.com/robpike/lisp
+;;
+;; which in turn were derived from code in "LISP 1.5 Programmer's Manual"
+;; by McCarthy, Abrahams, Edwards, Hart, and Levin, from MIT in 1962.
+;;
+
+
+(set! member (fn* (item list)
+                  "Return true if the specified item is found within the given list.
+
+See-also: intersection, union."
+
+                  (cond
+		   (nil? list) false
+		   (eq item (car list)) true
+		   true (member item (cdr list)))))
+
+(set! union (fn* (x y)
+      "Return the union of the two specified lists.
+
+See-also: intersection, member"
+      (cond
+       (nil? x) y
+       (member (car x) y) (union (cdr x) y)
+       true (cons (car x) (union (cdr x) y))
+       )))
+
+(set! intersection (fn* (x y)
+                        "Return the values common to both specified lists
+
+See-also: member, union."
+                        (cond
+		         (nil? x) nil
+		         (member (car x) y) (cons (car x) (intersection (cdr x) y))
+		         true (intersection (cdr x) y))))