[3-1] Why can't it deduce from (READ-FROM-STRING "foobar" :START 3)

      that the intent is to specify the START keyword parameter rather than
      the EOF-ERROR-P and EOF-VALUE optional parameters?

In Common Lisp, keyword symbols are first-class data objects.  Therefore,
they are perfectly valid values for optional parameters to functions.
There are only four functions in Common Lisp that have both optional and
keyword parameters (they are PARSE-NAMESTRING, READ-FROM-STRING,
WRITE-LINE, and WRITE-STRING), so it's probably not worth adding a
nonorthogonal kludge to the language just to make these functions slightly
less confusing; unfortunately, it's also not worth an incompatible change
to the language to redefine those functions to use only keyword arguments.
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