Basic Specifications with Subsorts

SIG-DECL        ::=   ... | SUBSORT-DECL
SUBSORT-DECL    ::=   EMBEDDING-DECL | ISO-DECL
EMBEDDING-DECL  ::=   embedding-decl SORT-LAYER+
SORT-LAYER      ::=   sort-layer SORT+
ISO-DECL        ::=   SORT-LAYER

BASIC-ITEM      ::=   ... | PRED-SORT-DEFN
PRED-SORT-DEFN  ::=   pred-sort-defn SORT VAR SORT FORMULA

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