examples: lisp — proper tail calls in eval

examples/lisp/test_tco.lisp

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+; test_tco.lisp — proper tail calls. Each form below would have blown
+; the C stack pre-TCO (segfault around ~25k recursive calls). The
+; counts here are deliberately past that line and across all three
+; tail positions: if-tail, begin-tail, let-tail.
+
+; if-tail: classic accumulator countdown.
+(define spin
+  (lambda (n a)
+    (if (= n 0) a (spin (- n 1) (+ a 1)))))
+(spin 30000 0)
+
+; begin-tail: last form of a begin block is the recursive call.
+(define bspin
+  (lambda (n a)
+    (if (= n 0) a (begin a (bspin (- n 1) (+ a 1))))))
+(bspin 30000 0)
+
+; let-tail: last form of a let body is the recursive call.
+(define lspin
+  (lambda (n a)
+    (if (= n 0) a (let ((m (- n 1))) (lspin m (+ a 1))))))
+(lspin 30000 0)
+
+; gcd: tail call from the else branch of an if (already tested by
+; lisp_test, kept here as a nice short demo).
+(define gcd (lambda (a b) (if (= b 0) a (gcd b (mod a b)))))
+(gcd 1071 462)