Preview character options for use as the fixed node separator in SWIGs.
Details
By default sep_opts() returns a named vector listing the built-in separators. These
built-in HTML entities are attached to nicenames for convenience. If the user provides
the nicename for a built-in entity to the entity argument, the function will return
the appropriate HTML code. If the user provides an HTML entity that does not appear in
the built-in menu, the input will be returned verbatim. Users will typically access
this latter functionality via qd_swig().