Properly negate impulses in chebfun.uminus().
Properly negate impulses in chebfun.uminus().

Surely negating all of them is the right thing...?  Even for the
higher-order impulses, they'll get hit because of the linearity of
differentiation...

ilovejs/chebfun,asgeirbirkis/chebfun,alshedivat/chebfun