-A field is like a marker but it defines a region rather than a
-point. Like a marker, a field is asocated with a buffer.
-The field mechanism uses the marker mechanism in the
-sense that its start and end points are maintained as markers
-updated in the usual way as the buffer changes.
+/* 1 if the OV is a cons cell whose car is a cons cell. */
+#define OVERLAY_VALID(OV) (OVERLAYP (OV))