argument pairs. Extensive modifications to the debugging
evaluator. Added "SECTION:" commentaries to clarify what happens
when, during double compilation. Renamed EVALIMP --> EVALIM.
Renamed EVAL --> XEVAL. Removed function evalcar. Defined
evalcar to scm_eval_car. Added explanation of "EVAL" symbols to
the beginning of the file. New procedure: scm_unmemocopy.
Added some global state variables needed by the debugging
evaluator: scm_ceval_ptr, last_debug_info_frame, debug_mode,
check_entry, check_apply, check_exit, debug_options and
evaluator_traps. New acro: undefine.