pipe ?fileId_var_r fileId_var_w?
              Create  a  pipe.   If fileId_var_r and fileId_var_r
              are specified, then pipe will set  the  a  variable
              named  fileId_var_r  to  contain  the fileId of the
              side of the pipe that was opened for  reading,  and
              fileId_var_w will contain the fileId of the side of
              the pipe that was opened for writing.

              If the fileId variables are not specified,  then  a
              list  containing  the  read  and  write  fileIdw is
              returned as the result of the command.

              This command is provided by Extended Tcl.