Merging strings and lists into lists

[bpj]

I have a file which contains mappings which par look like these (typically items in a list two or three mappings deep):

- rm: a
- rm: [a, b]
- rm: a
  ra: b
- rm: [a, b]
  ra: c
- rm: a
  ra: [b, c]
- rm: [a, b]
  ra: [c, d]

That is

• I have mappings with, among other keys, a key rm and sometimes a key ra.
• The values of those keys are either a string or a (flow) list of strings.
• Those strings aren’t actually single letters in alphabetical order, but their order is significant.
• I want to append the value of ra, if it exists, to the value of rm so that I have a list of the strings of both under rm and no ra.

Like so:

- rm: [a]
- rm: [a, b]
- rm: [a, b]
- rm: [a, b, c]
- rm: [a, b, c]
- rm: [a, b, c, d]

How can I do this? Do I need any extra pass(es) to ensure that the values of both keys are lists and to remove ra?

I would like to preserve the flow style of the lists, but that is not a necessity.