Task: Pos Tagging
Input (English): Sentence: -- On the basis of a differentiability assumption in function space , it is possible to prove that , for materials having the property that the stress is given by a functional of the history of the deformation gradients , the classical theory of infinitesimal viscoelasticity {{ is }} valid when the deformation has been infinitesimal for all times in the past . 
Word: is
Output (English): VERB