          incr varName ?increment?
               Increment the value stored in the variable  whose  name
               is   varName.   The  value  of  the  variable  must  be
               integral.  If increment  is  supplied  then  its  value
               (which  must  be  an  integer) is added to the value of
               variable varName;  otherwise 1  is  added  to  varName.
               The new value is stored as a decimal string in variable
               varName and also returned as result.
