1. New class Anchor: provides a mechanism for linking to arbitrary    content within a Document. 2. New function replace_special_characters: returns a copy of a string    with spaces and non-alphanumeric characters replaced by - and _. (Code    was originally in Section.id() method, which this replaces.)
1. New class Anchor: provides a mechanism for linking to arbitrary
   content within a Document.
2. New function replace_special_characters: returns a copy of a string
   with spaces and non-alphanumeric characters replaced by - and _. (Code
   was originally in Section.id() method, which this replaces.)

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