Questions on modular arithmetic functions in rat3a.lisp

Greetings!

1) The comments indicate that modulus is nil or positive.  Is it always
an integer?
2) The comments indicate that if modulus is non-nil, the arguments to
ctimes (et.al. ?) are numberp and less than modulus.  Are they in fact
integers?  Are they bounded from below?
3) GCL has a C implementation of ctimes which balances modular
arithmetic between -modulus/2 and modulus/2.  The generic lisp version
seems only to implement the upper bound.  Is there a reason for this?
Is the modular balancing necessary?
4) Why is bctimes/rem used in cbexpt instead of ctimes/mod?

Take care,
-- 
Camm Maguire			     		    camm at maguirefamily.org
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"The earth is but one country, and mankind its citizens."  --  Baha'u'llah