reserve x,y for Element of REAL;
reserve i,j,k for Element of NAT;
reserve a,b for Element of REAL;

theorem
  [*x,y*] in REAL implies y = 0
proof
  assume
A1: [*x,y*] in REAL;
  assume y <> 0;
  then [*x,y*] = (0,1) --> (x,y) by Def5;
  hence contradiction by A1,Th8;
end;
