Added TC for testing if palindrome

proofs/palindrome.dfy

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+// Proofs for Palindrome.dfy
+
+// Lemma 1:
+// If a is a palindrome, then either its length is <= 1
+// or its first and last elements are equal.
+lemma {:induction false} Palindrome_FirstLast(a: seq<int>)
+  ensures IsPalindrome(a) ==> (|a| <= 1 || a[0] == a[|a| - 1])
+{
+  // TODO replace this with a real proof.
+  assert false;
+}
+
+// Lemma 2:
+// If a is a palindrome and has length > 1, then its middle part
+// (drop first and last) is also a palindrome.
+lemma {:induction false} Palindrome_Middle(a: seq<int>)
+  ensures IsPalindrome(a) && |a| > 1 ==> IsPalindrome(a[1 .. |a| - 1])
+{
+  // TODO replace this with a real proof.
+  assert false;
+}
+

testcases/palindrome.dfy

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+// Palindrome.dfy
+// Simple palindrome tester on sequences of ints.
+
+// Recursive palindrome tester:
+// A sequence is a palindrome if:
+// - length 0 or 1: true
+// - otherwise: first == last AND the middle part is a palindrome.
+predicate {:induction false} IsPalindrome(a: seq<int>)
+  decreases |a|
+{
+  |a| <= 1 ||
+  (a[0] == a[|a| - 1] && IsPalindrome(a[1 .. |a| - 1]))
+}
+
+
+// Test 1:
+// If a is a palindrome, then either its length is <= 1
+// or its first and last elements are equal.
+method {:induction false} Test_Palindrome_FirstLast(a: seq<int>)
+{
+  assert IsPalindrome(a) ==> (|a| <= 1 || a[0] == a[|a| - 1]) by {
+    Palindrome_FirstLast(a);
+  }
+}
+
+// Test 2:
+// If a is a palindrome of length > 1, then its "middle"
+// (drop first and last element) is also a palindrome.
+method {:induction false} Test_Palindrome_Middle(a: seq<int>)
+{
+  assert IsPalindrome(a) && |a| > 1 ==> IsPalindrome(a[1 .. |a| - 1]) by {
+    Palindrome_Middle(a);
+  }
+}