“…The intuition leading to the play operator may be stated as follows: for a given input function, we look for an output function, starting value of which is given, and we do not change its value as long as the difference between input and output stays within a given boundary region. Several definitions of the play operator are possible, depending on the regularity of the input, the space in which the input and output attain their values and the boundary conditions (characteristics) , , , . It may be defined for sufficiently regular inputs and boundary conditions (characteristics), for example, with the Young or Kurzweil integral formalism , .…”