Prepending an string option

Hello, all

How can one prepend a Vim string option? I need to prepent
formatlistpat for Markdown. I have tried to combine `set'
and `execute()', but to no avail. `execute()' is documented
as returning the command output as a string, yet I can't
seem to call it as a function and use the return value in
`set':

set formatlistpat=prepend|execute('set formatlistpat?')

Also I fear I will lose escapes this way. Is it possible to
preserve them?

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