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A brace string is defined as a string consisting of the characters *()[] in which braces match correctly:
[brace-string] ::= [unit] || [unit] [brace-string]
[unit] ::= "" || "*" || "(" [brace-string] ")" || "[" [brace-string] "]"
This is a valid brace-string:
((())***[]**)****[(())*]*...