Compute a minimal annihilating operator of a bivariate hypergeometric sequence
  
  Calling sequence:
    MinimalAnnihilator( T , x , y , E );
  
  Input:
    T - hypergeometric sequence in x and y
    x - name
    y - name
    E - name of a shift operator ( Ef(x) = f(x+1) )    

  Output:
    A - difference operator of the form
      A = a[d] * E^d + ... + a[1] * E + a[0]
    where a[0], ..., a[d] are polynomials free of y, a[d] is nonzero,
    such that
      A T( x , y ) = 0
    and the order d is minimal.

  Reference:
    Polyakov S.P. On homogeneous Zeilberger recurrences.
    Advances in Applied Mathematics, 2008, vol. 40, no. 1, pp 1-7.