src/HOL/Set.thy

   "_setle"      :: "'a set => 'a set => bool"             ("op <=")
   "_setle"      :: "'a set => 'a set => bool"             ("(_/ <= _)" [50, 51] 50)
   "_setless"    :: "'a set => 'a set => bool"             ("op <")
   "_setless"    :: "'a set => 'a set => bool"             ("(_/ < _)" [50, 51] 50)
 
-syntax (symbols)
+syntax (xsymbols)
   "_setle"      :: "'a set => 'a set => bool"             ("op \<subseteq>")
   "_setle"      :: "'a set => 'a set => bool"             ("(_/ \<subseteq> _)" [50, 51] 50)
   "_setless"    :: "'a set => 'a set => bool"             ("op \<subset>")
   "_setless"    :: "'a set => 'a set => bool"             ("(_/ \<subset> _)" [50, 51] 50)
   "op Int"      :: "'a set => 'a set => 'a set"           (infixl "\<inter>" 70)