Red[
  author: "Nędza Darek"
]
str: "a1bcvcvba2mcbncva3z"
find str "a"

iterator: copy #()
find-next: func [
  str candidate
  /local

][
  iterator/candidate: candidate
  iterator/__series: str
  iterator/value: find iterator/__series iterator/candidate
  iterator/next:  func [][
    iterator/__series: next iterator/value
    iterator/value: find iterator/__series iterator/candidate
    iterator
  ]
  iterator
]
probe it: find-next str "a"
comment {
#(
    candidate: "a"
    __series: "a1bcvcvba2mcbncva3z"
    value: "a1bcvcvba2mcbncva3z"
    next: func [][
        iterator/__series: next iterator/value
        iterator/value: find iterator/__series iterator/candidate
        iterator
    ]
)
}
; comment section has only changing part of the map

probe it/next
comment {
     __series: "1bcvcvba2mcbncva3z"
    value: "a2mcbncva3z"
}
probe it/next
comment {
    __series: "2mcbncva3z"
    value: "a3z"
}