custom-prompt-variable is a compiled Lisp function in `
(custom-prompt-variable PROMPT-VAR PROMPT-VAL &optional COMMENT)
Prompt for a variable and a value and return them as a list.
PROMPT-VAR is the prompt for the variable, and PROMPT-VAL is the
prompt for the value. The %s escape in PROMPT-VAL is replaced with
the name of the variable.
If the variable has a `variable-interactive' property, that is used as if
it were the arg to `interactive' (which see) to interactively read the value.
If the variable has a `custom-type' property, it must be a widget and the
`:prompt-value' property of that widget will be used for reading the value.
If the variable also has a `custom-get' property, that is used for finding
the current value of the variable, otherwise `symbol-value' is used.
If optional COMMENT argument is non-nil, also prompt for a comment and return
it as the third element in the list.