"defined?" is essentially a module operation. SCM has only one
top-level environment and therefore it is clear which to use. But
Guile has many top-levels, so one has to say which one to use.
A convenient default choice is the `current' module, so it might be
worthwhile to put the "defined?" definition onto boot-9.scm. In any
case, I would like to remove it from eval.c.