read in string-snip%

Reads the snip's data from the given stream.

• (send a-string-snip read len f) -> void
     len : exact non-negative integer
     f : editor-stream-in% object

  The len argument specifies the maximum length of the text to be read. (When a text snip is written to a file, the very first field is the length of the text contained in the snip.) This method is usually invoked by the text snip class's read method.