Your custom particles type is not iterable, i.e. does not have start, next and done methods defined. See the manual for details of the iterable interface.
Note that Julia numbers are iterable, so the following works:
julia> findin([1,2,3], 2)
1-element Array{Int64,1}:
2
Thanks,
Jiahao Chen
Staff Research Scientist
MIT Computer Science and Artificial Intelligence Laboratory
