This program differentiates given expressions with respect to specified
unknowns. Expressions can be formed using brackets, the operators
 + - * /   and   @ (meaning 'to the power of')
and the functions
 sin() cos() tan() log() exp().
Unknowns are represented by Prolog atoms. The input should be given in
the form
         expression : unknown.
or
         expression : [unknown,unknown,...].
for multiple differentiation. For example,
         sin(x@2+1):x.
         x@4 + a*x@3 + b*x@2 + c*c + d : [x,x].
         x@y :[x,y,y].
are all acceptable. Don't forget the final dot!
To stop, type stop.
...............................................