Interesting! I think I am starting to get it. To double check, is running a grammar the same as parsing?
The Algol-68 was specified using two level grammar and if I understand it correctly, the declaration part constrained parsing of the statement part so that only valid expressions can be parsed. Parsed statements are valid in the semantic sense, i.e., the subscription can be applied only to array values and parser will reject subscription for scalar values.
To generate the grammar you need to execute some function. And this function depends on the part of input.