Another Python example

Again, from the reference manual:

dict_display ::= "{" [ key_datum_list ] "}"
key_datum_list ::= key_datum ( "," key_datum )* [","]
key_datum ::= expression ":" expression

In PGE, this becomes:

rule dict_display   { '{' <key_datum_list>? '}' }

rule key_datum_list { <key_datum> [ ',' <key_datum> ]* ','? }

rule key_datum      { <expression> ':' <expression> }