Read-string with pointer at start of initial value

[Whitfield Diffie]

    I want to call read-string with the pointer at the start of the
initial value so that I can delete it and type something else if it is
not right.  I am able to do something like this with read-file-name
because

(read-filename "PROMPT: " "~/" "DEFAULT-FILENAME" nil "INITIAL")

puts the mark after "~/" and before the initial input but I have found
no way of doing anything similar with read-string.


                                   Whit