Problem with the symbolic toolbox: unary operator - not implemented for Exp

[lo]

hello,

I am using the symbolic toolbox 1.1.0 in Octave 3.6.0,
and s being a symbol
I get the following

octave:16> differentiate(Exp(-s),s)
error: unary operator `-' not implemented for `ex' operands
error: evaluating argument list element number 1
error: evaluating argument list element number 1

on the opposite, for - not unary as in
differentiate(Exp(1-s),s)
it works as expected;

is there a work-around for this issue ?

another question:
is there a way to write the result of a symbolic calculation in a file ?

thanks a lot for the support !

regards,
Laurent