%% %CopyrightBegin%
%%
%% SPDX-License-Identifier: Apache-2.0
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[;1m  union(Set1, Set2)[0m

  Returns the union of [;;4mSet1[0m and [;;4mSet2[0m.

[;1mExamples[0m

    1> S1 = sofs:set([a,b,c]).
    2> S2 = sofs:set([c,d,1,2,3]).
    3> S3 = sofs:union(S1, S2).
    4> sofs:to_external(S3).
    [1,2,3,a,b,c,d]
