uplevel level command command ...
      All of the command arguments are concatenated as if 
      they had been passed to concat and the result is  
      evaluated in the variable context indicated by level. 
      Uplevel returns the result of that evaluation.  If 
      level is zero, then top-level context is used (all 
      variable names refer to global variables).  If level is 
      a positive number, then it is treated as a stack level: 
      1 refers to the topmost active procedure, 2 to the 
      procedure it called, and so on. If level is a negative 
      number, then it is treated as a level relative to the 
      current procedure.  For example, a level of -1 refers 
      to the procedure that called the one invoking the 
      uplevel command (which is top-level if the procedure 
      invoking uplevel is at level 1).  The uplevel command 
      causes the invoking procedure to disappear from the 
      procedure calling stack while the command is being 
      executed.  For example, suppose procedure x is at level 
      3 and invokes the command    

      uplevel -1 {set a 43; c}     

      where c is another Tcl procedure.  The set command will 
      modify variable a in x's caller, and c will execute at 
      level 3, as if called from x's caller.  If it in turn 
      executes the command     

      uplevel -1 {set a 42}     

      then the set command will modify the same variable a in 
      x's caller:  the procedure x does not appear to be on 
      the call stack when c is executing.  The command ``info 
      level'' may be used to obtain the level of the current 
      procedure.  Uplevel makes it possible to implement new 
      control constructs as Tcl procedures (for example, 
      uplevel could be used to implement the while construct 
      as a Tcl procedure).