Parentheses sequences in lexicographical order
Perl 6, 36 bytes



{grep {try !.EVAL},[X~] &lt;[ ]&gt;xx$_*2}


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Finds all lexographically sorted combinations of \$2n\$ []s and filters the ones that EVAL correctly. Note that all valid combinations (even stuff like [][]) evaluate to [] (which is falsey, but we not it (!) to distinguish from try returning Nil)

Explanation:

{                                  }  # Anonymous code block
                        &lt;[ ]&gt;         # Create a list of ("[", "]")
                             xx$_*2   # Duplicate this 2n times
                   [X~]               # Find all possible combinations
 grep {          },                   # Filter from these
            .EVAL                     # The EVAL'd strings
       try !                          # That do not throw an error