Add elisp/leap

elisp/leap/README.md

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+# Leap
+
+Given a year, report if it is a leap year.
+
+The tricky thing here is that a leap year in the Gregorian calendar occurs:
+
+```plain
+on every year that is evenly divisible by 4
+  except every year that is evenly divisible by 100
+    unless the year is also evenly divisible by 400
+```
+
+For example, 1997 is not a leap year, but 1996 is.  1900 is not a leap
+year, but 2000 is.
+
+If your language provides a method in the standard library that does
+this look-up, pretend it doesn't exist and implement it yourself.
+
+## Notes
+
+Though our exercise adopts some very simple rules, there is more to
+learn!
+
+For a delightful, four minute explanation of the whole leap year
+phenomenon, go watch [this youtube video][video].
+
+[video]: http://www.youtube.com/watch?v=xX96xng7sAE
+## Source
+
+JavaRanch Cattle Drive, exercise 3 [http://www.javaranch.com/leap.jsp](http://www.javaranch.com/leap.jsp)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

elisp/leap/leap-test.el

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+;;; leap-test.el --- Tests for Leap exercise (exercism)
+
+;;; Commentary:
+
+;;; Code:
+(load-file "leap.el")
+
+(ert-deftest vanilla-leap-year ()
+  (should (leap-year-p 1996)))
+
+(ert-deftest any-old-year ()
+  (should-not (leap-year-p 1997)))
+
+(ert-deftest non-leap-even-year ()
+  (should-not (leap-year-p 1997)))
+
+(ert-deftest century ()
+  (should-not (leap-year-p 1900)))
+
+(ert-deftest exceptional-century ()
+  (should (leap-year-p 2000)))
+
+(provide 'leap-test)
+;;; leap-test.el ends here

elisp/leap/leap.el

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+;;; leap.el --- Leap exercise (exercism)
+
+;;; Commentary:
+
+;;; Code:
+
+(provide 'leap)
+
+(defun leap-year-p (year)
+"Return whether a given year is a leap-year."
+  (when (= (mod year 4) 0)
+    (if (= (mod year 100) 0)
+        (when (= (mod year 400) 0) t)
+      t)))
+
+;;; leap.el ends here