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

  Returns [;;4mtrue[0m if [;;4mSet1[0m is a subset of [;;4mSet2[0m; otherwise, returns [;;4m[0m
  [;;4mfalse[0m.

    1> S1 = sofs:set([2,4,6]).
    2> S2 = sofs:set([1,2,3,4,5,6]).
    3> sofs:is_subset(S1, S2).
    true
    4> sofs:is_subset(S2, S1).
    false
    5> sofs:is_subset(S1, S1).
    true
    6> S3 = sofs:relation([{1,a},{2,b}]).
    7> S4 = sofs:relation([{1,a}]).
    8> sofs:is_subset(S4, S3).
    true
    9> sofs:is_subset(S3, S1).
    ** exception error: type_mismatch
         in function  sofs:is_subset/2
