Compiler issues:

1: SETQ

The ORBIT compiler goes through a process of assignment conversion
making the use of SETQ much less efficient than would otherwise be
possible.  However, the only reason this is in the compiler is to
support CALL-WITH-CURRENT-CONTINUATION (basically the ability to copy
the stack.)  We do not intend to support CALL-WITH-CURRENT-CONTINUATION,
therefore we will remove this feature from the compiler.  We do not
believe that this will be a difficult change.

2: lexical goto

The dynamic state of the program will be kept on a stack similar to the
current special pdl.  It will contain dynamic bindings, pointers to
unwind protect code segments, and indices for active frames in the main
stack.  When a lexical goto requires an adjustment in the dynamic state,
the compiler will output instructions to unwind the stack.  

3: catch and throw

Catch tags will be kept on the extra stack mentioned above.  Therefore,
finding a catch tag will take a number of memory cycles proportional to
its depth (which is close to the number of dynamic bindings.)  Both
machines then unwind the extra stack by doing about the same number of
cycles.  Therefore, the new machine will do about 1.5 times as many
memory cycles as the lambda.  A catch on the new machine will requires
pushing two words on the extra stack, compared to 32 instructions on a
lambda.