reserve f for PartFunc of REAL-NS 1,REAL-NS 1;
reserve g for PartFunc of REAL,REAL;
reserve x for Point of REAL-NS 1;
reserve y for Real;

theorem
  f=<>*g & x=<*y*> implies (f is_differentiable_in x iff g
  is_differentiable_in y) by Th7,Th8;
