read-one-special in readable-snip<%>

• (send a-readable-snip read-one-special index source line column position) -> three values: an arbitrary value, an exact non-negative integer, and a boolean
     index : exact non-negative integer
     source : value
     line : exact non-negative integer or #f
   column : exact non-negative integer or #f
   position : exact non-negative integer or #f

  The index argument is 0 the first time this method is called for a particular output stream. If the last result value is #f, then the method is called again to obtain additional results, and index is incremented for each call.

  The remaining arguments are the same as the arguments to a procedure returned by a custom input port's read-string-proc; see Custom Ports, section 11.1.6 in PLT MzScheme: Language Manual for details. The first two results are also the same as the results from a read-string-proc-produced procedure.

  If the method raises exn:special-comment, the exception is used in the same way as when a custom import port raises the exception. The method will not be called for additional results in that case.