function YESNO = isinterval(F)
%ISINTERVAL (overloaded)
F = flatten(F);
if isempty(F.clauses)
YESNO = 1;
else
i = 1;
while (i<=length(F.clauses)) & YESNO
Fi = F.clauses{i};
YESNO = YESNO & is(Fi.data,'interval');
i = i+1;
end