(defun positions-if (predicate sequence &key (start 0) end key)
  "Return a fresh list of all positions in `sequence` that satisfy `predicate`.

  Like `cl:position-if`, but returns a list of all the results.

  Example:

    (positions-if #'upper-case-p \"aBCdeF\")
    ; =>
    (1 2 5)

  "
  (let ((pos start))
    (nreverse (reduce (lambda (result value)
                        (prog1 (if (funcall predicate value)
                                 (cons pos result)
                                 result)
                          (incf pos)))
                      sequence
                      :start start
                      :end end
                      :key key
                      :initial-value nil))))