Initial data commit

Task/Mutual-recursion/BASIC/mutual-recursion.basic

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+DECLARE FUNCTION f! (n!)
+DECLARE FUNCTION m! (n!)
+
+FUNCTION f! (n!)
+    IF n = 0 THEN
+        f = 1
+    ELSE
+        f = m(f(n - 1))
+    END IF
+END FUNCTION
+
+FUNCTION m! (n!)
+    IF n = 0 THEN
+        m = 0
+    ELSE
+        m = f(m(n - 1))
+    END IF
+END FUNCTION