"TAIL" is the node that defines the end of a non-capturing parenthetical group or an if-then parenthetical group (like (?(1)a|b)). That's about all. It gets optimized out for non-capturing parens if there is no "BRANCH" found (that is, no "|" metacharacter), so that something like /A(?:B)C/ can be optimized to an "EXACT" node matching "ABC".

Basically, "TAIL" takes the place of having another "indented" layer.

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;