Tests if a one-character string is some kind of bracket.
The call isbra s tests whether the first character of string s (normally
it is the only character) is a bracket, meaning an opening or closing
parenthesis, square bracket or curly brace.
- FAILURE CONDITIONS
Fails if the string is empty.
- SEE ALSO
isalnum, isalpha, isnum, issep, isspace, issymb.