Description: Define the subclass
relationship. Exercise 9 of [TakeutiZaring] p. 18.
For example, {1, 2} ⊆ {1, 2, 3} (ex-ss 27284). Note that
𝐴
⊆ 𝐴 (proved in
ssid 3624). Contrast this relationship with the
relationship 𝐴 ⊊ 𝐵 (as will be defined in df-pss 3590). For a more
traditional definition, but requiring a dummy variable, see dfss2 3591.
Other possible definitions are given by dfss3 3592, dfss4 3858, sspss 3706,
ssequn1 3783, ssequn2 3786, sseqin2 3817, and ssdif0 3942. (Contributed by NM,
27-Apr-1994.) |