What is "[+]" in xemacs regular expression?
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In (my) XEmacs 21.4,  describe-variable for gdb-prompt-pattern gives:

    Value: "^>\\|^(.*gdb[+]?) *\\|^---Type <return> to.*--- *"

What is the "[+]" in there?  Does it just mean "the + character"?  If so, why not use "\\+"?