Procedure to return the logical union of the two specified lists.  Any duplicate elements are removed.

Usage:
  union lista listb

Example:
  package require Tclx
  union {a b c d e} {d e d f g c} ;# ==> a b c d e f g

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This functionality exists in [tcllib] in the [struct::set] package.
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[Category Command], a part of [Tclx]