nice ?priorityincr?
              Change  or return the process priority.  If priori-
              tyincr  is  omitted,  the   current   priority   is
              returned.  If priorityincr is positive, it is added
              to the current  priority  level,  up  to  a  system
              defined maximum (normally 19),

              Negative  priorityincr values cumulatively increase
              the program's priority down  to  a  system  defined
              minimum  (normally  -19);  increasing priority with
              negative niceness values will  only  work  for  the
              superuser.

              The new priority is returned.

              This command is provided by Extended Tcl.