Jan Vroonhof <vroonhof(a)math.ethz.ch&gt; writes: Not if the lvalue is supposed to be read-only. I prefer a crash to silent problems. If we agree that we don't write to purespace, then we should not use XC[AD]R as lvalues, regardless of whether the coredump will happen if we slip. Given how hard it is to maintain something as large as XEmacs, I would prefer the coredump to happen. What function call? Have you read my proposal on how to transform the code that uses XCAR/XCDR as lvalues? (hint: I don't propose reverting to Fsetcar().)