A.2 Basic Specifications with Subsorts

  SORT-ITEM        ::= ... | SUBSORT-DECL | ISO-DECL | SUBSORT-DEFN

  SUBSORT-DECL     ::= subsort-decl SORT+ SORT
  ISO-DECL         ::= iso-decl SORT+
  SUBSORT-DEFN     ::= subsort-defn SORT VAR SORT FORMULA

  ALTERNATIVE      ::= ... | SUBSORTS
  SUBSORTS         ::= subsorts SORT+

  ATOM             ::= ... | MEMBERSHIP
  MEMBERSHIP       ::= membership TERM SORT

  TERM             ::= ... | CAST
  CAST             ::= cast TERM SORT