Thanks for catching my error above. I think, its is really necessary to provide a rule that there must not be a whitespace between the two {{ and the two }}. Unfortunately, the PEG parser generator does not provide such a command. I changed Parser/parser.c by hand to accept '{' '{' star_named_expressions '}' '}' only if there is no whitespace between the two opening and the two closing braces. In this case:
{{ {a,b,c} }}is a frozen set of a set (thus invalid){ {{a,b,c}} }is a set of a frozen set (valid){{ {a,b,c}} }is a set of a set of a set (invalid){{{a,b,c}}}is parsed as a frozen set of a set (thus invalid) as I would usually expect.{{{a,b}},{{c}}}is parsed as a set of two frozen sets (valid).