/* INPUT AND OUTPUT:

  Reading and writing characters (= one letter terms):

    get_char(X)
    put_char(X)

  Reading and writing strings (= any term):

    read(X)
    write(X)

  If X is uninstantiated, next char or string (string ended by . and
  newline) will be read and X will be instantiated. If X is already
  instantiated, the goal will be satisfied iff X matches what is read.
  These goals will only succed once.

  The predicate nl flushes output, adding a newline.

  File access:

    open(X,Y,Z) [X filename, Y is read or write, Z is "filehandle"]
    close(Z)
    set_input(Z)
    set_output(Z)
    current_input(Z)
    current_output(Z)

  Reading and writing acts on "current stream", initially keyboard and
  screen. This can be changed using filehandles, obtained by open. The
  change is done by set_input and set_outpot. The current_input and
  current_output predicates get filehandles for current streams.

*/