file is the PostScript operator used to open files
for reading or writing. If opening is successful the
appropriate streamtype is returned. If opening fails
/FileOpenError is raised.
If the file name contains path information "/" the
operator tries to open the specified file directly
otherwise if the file should be opened for reading
the operator uses the searchpath to locate the file.
string1 is the file name string2 is a modifier (r) or
(w) which specifies if the file should be opened for
reading or writing.
string1 string2 file -> ostreamtype