The %onprompt() mechanism is not documented
@sgougeon)
Reported by Samuel GOUGEON (BUG DESCRIPTION:
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There is no help page describing the %onprompt() customizable special function.
HOW TO REPRODUCE THE BUG:
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help %onprompt
OTHER INFORMATION:
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Searching for the "%onprompt" key on the online manual yields an error: See the bug 14921