The vars argument expects a list or vector with named elements. 
  Every element name corresponds to the widget name which will be updated 
  with the supplied element value.
The vector, matrix, and data widgets can be updated in 
  several ways. If more than one name is specified for the names argument 
  of these widgets, each element is treated like an entry widget.
If however, a single name describes any of these three widgets, the entire 
  widget can be updated by passing an appropriately sized object.
Alternatively, any element can be updated by appending its index in square brackets 
  to the end of the name. The data widget is indexed differently 
  than the matrix widget by adding "d" after the brackets. This tweak is necessary 
  for the internal coding (bookkeeping) of PBS Modelling. Example: "foo[1,1]d".