+ /* For the data tracing facility, we need to insert `trace' bytecodes
+ before each data fetch; this records all the memory that the
+ expression touches in the course of evaluation, so that memory will
+ be available when the user later tries to evaluate the expression
+ in GDB.
+
+ Setting the flag 'tracing' to non-zero enables the code that
+ emits the trace bytecodes at the appropriate points. */