New convenience functions in seq.el

Functions to access the first or all but the first elements of
sequences have been repeatedly asked for (the last occurrence being
https://github.com/NicolasPetton/seq.el/issues/9).

* lisp/emacs-lisp/seq.el (seq-first, seq-rest): New functions.
* test/lisp/emacs-lisp/seq-tests.el (test-seq-first, test-seq-rest):
New tests for seq-first and seq-rest.

lisp/emacs-lisp/seq.el

@@ -4,7 +4,7 @@
 
 ;; Author: Nicolas Petton <nicolas@petton.fr>
 ;; Keywords: sequences
-;; Version: 2.20
+;; Version: 2.21
 ;; Package: seq
 
 ;; Maintainer: emacs-devel@gnu.org
@@ -110,6 +110,14 @@ name to be bound to the rest of SEQUENCE."
   "Return the number of elements of SEQUENCE."
   (length sequence))
 
+(defun seq-first (sequence)
+  "Return the first element of SEQUENCE."
+  (seq-elt sequence 0))
+
+(defun seq-rest (sequence)
+  "Return a sequence of the elements of SEQUENCE except the first one."
+  (seq-drop sequence 1))
+
 (cl-defgeneric seq-do (function sequence)
   "Apply FUNCTION to each element of SEQUENCE, presumably for side effects.
 Return SEQUENCE."

test/lisp/emacs-lisp/seq-tests.el

@@ -424,5 +424,17 @@ Evaluate BODY for each created sequence.
     (should (eq (seq-into vec 'vector) vec))
     (should (eq (seq-into str 'string) str))))
 
+(ert-deftest test-seq-first ()
+  (let ((lst '(1 2 3))
+        (vec [1 2 3]))
+    (should (eq (seq-first lst) 1))
+    (should (eq (seq-first vec) 1))))
+
+(ert-deftest test-seq-rest ()
+  (let ((lst '(1 2 3))
+        (vec [1 2 3]))
+    (should (equal (seq-rest lst) '(2 3)))
+    (should (equal (seq-rest vec) [2 3]))))
+
 (provide 'seq-tests)
 ;;; seq-tests.el ends here