REDUCE Portable Standard Lisp (PSL)

REDUCE is an interactive system for general algebraic computations of
interest to mathematicians, scientists and engineers.

PSL was the original implementation of Standard Lisp, but now contains
many more facilities. It is quite efficient in its use of both space and
time, and has been optimized for algebraic computation. All PSL versions
of REDUCE are distributed with sufficient PSL support to run on the given
computing system. PSL is supported on many architectures and is an ideal
system for those wanting to run REDUCE as a stand-alone system. The
principal developer of PSL before it became Open Source was the Konrad
Zuse Center, Berlin (ZIB).

It is often used as an algebraic calculator for problems that are possible
to do by hand. However,  REDUCE is designed to support calculations that
are not feasible by hand.