-DEFUN ("substring", Fsubstring, Ssubstring, 2, 3, 0,
+/* Check that ARRAY can have a valid subarray [FROM..TO),
+ given that its size is SIZE.
+ If FROM is nil, use 0; if TO is nil, use SIZE.
+ Count negative values backwards from the end.
+ Set *IFROM and *ITO to the two indexes used. */
+
+static void
+validate_subarray (Lisp_Object array, Lisp_Object from, Lisp_Object to,
+ ptrdiff_t size, EMACS_INT *ifrom, EMACS_INT *ito)
+{
+ EMACS_INT f, t;
+
+ if (INTEGERP (from))
+ {
+ f = XINT (from);
+ if (f < 0)
+ f += size;
+ }
+ else if (NILP (from))
+ f = 0;
+ else
+ wrong_type_argument (Qintegerp, from);
+
+ if (INTEGERP (to))
+ {
+ t = XINT (to);
+ if (t < 0)
+ t += size;
+ }
+ else if (NILP (to))
+ t = size;
+ else
+ wrong_type_argument (Qintegerp, to);
+
+ if (! (0 <= f && f <= t && t <= size))
+ args_out_of_range_3 (array, from, to);
+
+ *ifrom = f;
+ *ito = t;
+}
+
+DEFUN ("substring", Fsubstring, Ssubstring, 1, 3, 0,