to the end of the file.
          The "append mode" can be turned off while the port is in use
          see fcntl

     The following additional characters can be appended:

     b
          Open the underlying file in binary mode, if supported by the
          system.  Also, open the file using the binary-compatible
          character encoding "ISO-8859-1", ignoring the default port
          encoding.

     +
          Open the port for both input and output.  E.g., `r+': open an
          existing file for both input and output.

     0
          Create an "unbuffered" port.  In this case input and output
          operations are passed directly to the underlying port
          implementation without additional buffering.  This is likely
          to slow down I/O operations.  The buffering mode can be
          changed while a port is in use see setvbuf

     l
          Add line-buffering to the port.  The port output buffer will
          be automatically flushed whenever a newline character is
          written.

     In theory we could create read/write ports which were buffered in
     one direction only.  However this isn't included in the current
     interfaces.  If a file cannot be opened with the access requested,
     `open-file' throws an exception.

