theorem Th34:
  for f being array of X st z in dom f & z <> x & z <> y
  holds Swap(f,x,y)/.z = f/.z
  proof
    let f be array of X;
    assume
A1: z in dom f & z <> x & z <> y;
    dom Swap(f,x,y) = dom f by FUNCT_7:99;
    hence Swap(f,x,y)/.z = Swap(f,x,y).z by A1,PARTFUN1:def 6
    .= f.z by A1,Th33 .= f/.z by A1,PARTFUN1:def 6;
  end;
