Whizbang!
Roy A. Sykes · ACM SIGAPL APL Quote Quad · 1996
Originally, APL had only one set function, membership ( L∈R ). Defined as [EQUATION], it simply flagged whether each element of its left argument was found anywhere in its right argument. With the introduction of nested arrays, the definition was refined to [EQUATION] so that nested items could be be searched as well as simple elements. Users immediately noticed the ∪ and ∩ keys on the APL keyboard and wondered why they were not implemented as set union and intersection. Several implementations did include them, but APL2 implemented only without ( L ~ R ).