Add boolean chain approach for leap

exercises/practice/leap/.approaches/boolean-chain/content.md

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+# Boolean chaining
+
+```pyret
+fun leap(year):
+  fun year-is-divisible-by(divisor):
+    num-equal(num-modulo(year, divisor), 0)
+  end
+
+  year-is-divisible-by(4) and (not(year-is-divisible-by(100)) or year-is-divisible-by(400))
+end
+```
+
+This approach uses the Boolean operators `and` and `or` to chain together values from two Boolean expressions, creating a Boolean value.
+The `and` operator returns `true` when both sides are true but `false` otherwise.
+The `or` operator returns `false` when both sides are false but `true` otherwise.
+
+|   n1  |   n2  | n1 OR n2 | n1 AND n2 |
+| ----- | ----- | -------  | --------- |
+| false | false |    false |     false |
+| false | true  |     true |     false |
+| true  | false |     true |     false |
+| true  | true  |     true |      true | 
+
+Both operators can short-circuit which means in some scenarios, the operators don't evaluate the expression to the right.
+For `and`, if the left side produces `false`, that value is returned immediately.
+For `or`, if the left side produces `true`, that value is returned immediately.
+
+We can now test if a year is evenly divisible by 4, 100, and 400. 
+All leap years are divisible by 4 but not by 100 unless they're also divisible by 100.
+
+| year | year % 4 == 0 | year % 100 != 0 | year % 400 == 0 | is leap year |
+| ---- | ------------- | --------------- | --------------- | ------------ |
+| 2020 |          true |            true | (not evaluated) |         true |
+| 2019 |         false | (not evaluated) | (not evaluated) |        false |
+| 2000 |          true |           false |            true |         true |
+| 1900 |          true |           false |           false |        false |

exercises/practice/leap/.approaches/boolean-chain/snippet.txt

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+PLACEHOLDER

exercises/practice/leap/.approaches/config.json

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+{
+  "introduction": {
+    "authors": [
+      "BNAndras"
+    ]
+  },
+  "approaches": [
+    {
+      "uuid": "2f53896f-b3e7-4c7d-be3a-a3342087826e",
+      "slug": "boolean-chain",
+      "title": "Boolean Chaining",
+      "blurb": "Use operators to check boolean values in a chain",
+      "authors": [
+        "BNAndras"
+      ]
+    }
+  ]
+}

exercises/practice/leap/.approaches/introduction.md

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+# Introduction
+
+There are two approaches highlighted here for his exercise.
+Both involve checking if a year is evenly divisible by 4, 100, and 400 using [num-modulo][num-modulo].
+
+## General guidance
+
+Regardless of the approach chosen, this exercise requires students to use Boolean logic to decide if a given year is a leap year.
+
+## Approach: Boolean chaining
+
+```pyret
+fun leap(year):
+  fun year-is-divisible-by(divisor):
+    num-equal(num-modulo(year, divisor), 0)
+  end
+
+  year-is-divisible-by(4) and (not(year-is-divisible-by(100)) or year-is-divisible-by(400))
+end
+```
+
+For more information, check the [Boolean chain approach][approach-boolean-chain].
+
+[num-modulo]: https://pyret.org/docs/latest/numbers.html#%28part._numbers_num-modulo%29
+[approach-boolean-chain]: https://exercism.org/tracks/pyret/exercises/leap/approaches/boolean-chain