+ *
+ * The @x_reallocv@ function adjusts a block which currently has
+ * space for @on@ elements each of size @sz@, so that it now has
+ * enough space for @n@ elements, preserving the initial @min(n,
+ * on)@ elements. The @x_realloc@ function is the same, but
+ * with @sz@ fixed equal to 1, and @n@ and @on@ renamed to @sz@
+ * and @osz@ for historical reasons.