vtkArrayCalculator constant vector operation

A simple Python vtkArrayCalculator question: how can I specify a constant vector c in SetFunction so that I can do vector operation like U + c for an existing vector field U? I introduce U using AddVectorArrayName("U").

Edit: I found out one way: U + 0.1*iHat + 0.2*jHat + 0.3*kHat, is there another way?