Implement iterator over subsets of array

General Usage
9

How about something like this

struct Atom
  name::String
  molecule::Int
end

atoms = [ Atom("O",1), Atom("H",1), Atom("H",1), 
                 Atom("O",2), Atom("H",2), Atom("H",2) ]

struct EachMolecule 
    atoms::Vector{Atom}
end

eachmolecule(atoms) = EachMolecule(atoms)

function Base.iterate(em::EachMolecule, state = 1)
    r0 = state
    r0 > length(em.atoms) && return nothing
    m0 = em.atoms[r0].molecule
    r1 = r0
    while r1 <= length(em.atoms)
        em.atoms[r1].molecule != m0 && return (r0:r1 - 1, r1)
        r1 += 1
    end

    return (r0:r1 - 1, r1)
end

julia> for molecule in eachmolecule(atoms)
           println(molecule)
       end
1:3
4:6
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