Syntax: PRINT [+] [ filename ] [ 'format' ] { vector_list }
or      PRINT [+] [ filename ] [ 'format' ] < vector_list >

   Print the values of the vectors in vector_list to the terminal, or,
if filename is specified, to that file. If a file is specified, each
line of the header starts with a `#', so the file can be read without
using the LINES command. The header is not printed if the variable
print_noheader is defined and non-zero.

   With the optional `+' the vectors are appended to the file, otherwise
it is overwritten.

   The optional format string is of the type accepted by the C
function `printf', and you should see the manual or a book on C for
more details. Basically, the format string is copied to the output
with output specified by % signs. Each line is NOT terminated by a
newline, you have to write them explicitly.

   For example,
      SET x=1,10 SET y=x**2
      PRINT file '%10f (%10.2e)\n' { x y }
will produce
#        x            y
#
  1.000000 (  1.00e+00)
  2.000000 (  4.00e+00)
  3.000000 (  9.00e+00)
(etc.)
and
      PRINT file '%g ' { x }
#         x
#
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