X<Y, Y<Z -> X<Z.
X<X -> [].

X=<X.
X<Y -> X=<Y.
X=<Y -> X=Y,X<Y.


a<t.
b<t.
a=b -> [].
X=a, X=b, X=t.
a<b -> [].
b<a -> [].


l>=a.
l>=b.

l>=t -> [].


precedence([l,t,a,b]).