Stack filtered elements on top of iterator elements

stack.jl

struct Stack{I,F}
    iter::I
    f::F
end
import Base.iterate


function iterate(S::Stack) 
    ϕ = iterate(S.iter)
    ϕ === nothing && return nothing
    x, state = ϕ
    while !S.f(x)
        ϕ = iterate(S.iter, state)
        ϕ === nothing && return nothing
        x, state = ϕ
    end
    iterate(S, (x, state))
end


function iterate(S::Stack, (header, state))
    ϕ = iterate(S.iter, state)
    ϕ === nothing && return nothing
    x, state = ϕ
    while S.f(x)
        header = x
        ϕ = iterate(S.iter, state)
        ϕ === nothing && return nothing
        x, state = ϕ
    end
    (header, x), (header, state)
end
Base.IteratorSize(::Stack) = Base.SizeUnknown()
S = Stack(1:20, x -> x in [3, 10, 15])
Base.eltype(S::Stack) = Tuple{eltype(S.iter),eltype(S.iter)}
Base.IteratorEltype(::Stack{I}) where {I} = Base.IteratorEltype(I)
iterate(S)
collect(S)