We extend the language of balanced parentheses from 2.3 with two symbols: [ and ]. [ corresponds to exactly two normal opening parentheses and ] corresponds to exactly two normal closing parentheses....

We extend the language of balanced parentheses from 2.3

with two symbols: [ and ]. [ corresponds to exactly two normal opening parentheses and ] corresponds to exactly two normal closing parentheses. A string of mixed

parentheses is legal if and only if the string produced by replacing [ by (( and ]

by )) is a balanced parentheses sequence. Examples of legal strings are