SORT-DECL ::= sort-decl SORT+ SORT ::= TOKEN-ID
A sort declaration SORT-DECL is written:
S1,...,Sn
It declares each of the sorts in the list S1, ..., Sn. The single declaration of several sorts `sorts S1,...,Sn;' is equivalent to the sequence of declarations of single sorts `sorts S1; ...; Sn;'.