Sorry master, shouldn’t it be instead:
in(x) is equivalent to y -> y in x, i.e. in(x)(y) == y in x
That means that: in(x).(y) will be like: y .∈ (x,)
In the OP example, the following two are equivalent:
( in([1, 2]).([1, 2, 3]) ) == ( [1, 2, 3] .∈ ([1, 2],) ) # true