I am sure as hell no Inform wizard, but here is a piece of code
that should do what you want:
Class Stairwell
with description [;
print "The grey, cold, bleak, featureless, echoing
stairwell \
is one of the infamous design ~features~ of the
CS \
department. The stairs lead ";
if ((self).u_to~=0)
{ print "up";
if ((self).d_to~=0) print " and "; }
if ((self).d_to~=0) print "down"; ".";
],
has light;
Martin