ASCIIMath parsing: {: :} within {: :}

Created by: karbuncle

It seems to cause Math processing error whenever the pair of brackets {: :} is wrapped in another pair of brackets {: :}, and is the only thing in that bracket.

Examples:

not ok {: {::} :}
not ok {: {:     :}    :}

ok {: {:  something :} :}
ok {: {:  :} something :}