ins(E)=create -> [].
del(E)=create -> [].
dmin(E)=create -> [].

dmin(E1)=del(E2) -> [].
ins(E1)=del(E2) -> [].
ins(E1)=dmin(E2) -> [].



precedence([dmin,ins,del,create,inf,undef,=<,<]).