theorem Th51:
  (i1 ';' i2 ';' i3 ';' i4).2 = IncAddr(i3,2)
 proof
  LastLoc(i1 ';' i2 ';' i3) = 3 by Th31;
  hence (i1 ';' i2 ';' i3 ';' i4).2 = (i1 ';' i2 ';' i3).2 by Th23,Th28
        .= IncAddr(i3,2) by Th50;
 end;
