Is in(a,b) defined automatically by making b iterable and defining ==(a1::T, a2::T) where T is typeof(a) and typeof(b[i])?
in is defined as
in(x, itr) = any(y -> y == x, itr)
any is defined as
function any(f, itr) for x in itr f(x) && return true end return false end
I think it would be handy if a dependency tree of all these Base functions often overloaded was documented.
Perhaps, the documentation is pretty good here though imo:
Determine whether an item is in the given collection, in the sense that it
is == to one of the values generated by iterating over the collection.