12.0.14 `print_xv_form'
-----------------------

print_xv_form(S)
     :  It transforms S to a gif file and previews the file by xv.

print_xv_form(S | INPUT=key0,FORMAT=key1)
     : This function allows optional variables   INPUT, FORMAT
Example 0:
      print_xv_form(newmat(2,2,[[x^2,x],[y^2-1,x/(x-1)]]));
   Example 1:
      print_xv_form(x+y | format="png");
    If the optional variable format="png" is set,   png format will be
used to generate an input for xv.

   References:

      `print_tex_form' ,  `print_gif_form'

