SORT-DECL ::= sort-decl SORT+ ! SORT ::= TOKEN-ID
A sort declaration SORT-DECL is written:
S_1,..., S_n
It declares each of the sorts in the list S1, ..., Sn. The single declaration of several sorts in `sorts S_1,..., S_n;' is equivalent to the sequence of declarations of single sorts in `sorts S_1; ... S_n;'.