theorem
  for A be non empty set, Y, Z be bound_QC-variable of Al,
   V1, V2 be Element of Valuations_in(Al,A)
  ex v being Element of Valuations_in(Al,A) st
   (for x being bound_QC-variable of Al st x <> Y holds v.x = V1.x) &
   v.Y = V2.Z by Lm3;
