%% %CopyrightBegin%
%%
%% SPDX-License-Identifier: Apache-2.0
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[;1m  family_field(Family1)[0m

  If [;;4mFamily1[0m is a family and [;;4mFamily1[0m[i] is a binary relation for
  every i in the index set of [;;4mFamily1[0m, then [;;4mFamily2[0m is the
  family with the same index set as [;;4mFamily1[0m such that [;;4mFamily2[0m[i]
  is the field of [;;4mFamily1[0m[i].

[;1mExamples[0m

    1> FR = sofs:from_term([{a,[{1,a},{2,b},{3,c}]},{b,[]},{c,[{4,d},{5,e}]}]).
    2> F = sofs:family_field(FR).
    3> sofs:to_external(F).
    [{a,[1,2,3,a,b,c]},{b,[]},{c,[4,5,d,e]}]

  [;;4mfamily_field(Family1)[0m is equivalent to [;;4m[0m
  [;;4mfamily_union(family_domain(Family1), family_range(Family1))[0m.
