reserve X,Y for set, p,x,x1,x2,y,y1,y2,z,z1,z2 for object;
reserve f,g,h for Function;

theorem
  f = {[x,y]} implies f.x = y
proof
  assume f = {[x,y]};
  then [x,y] in f by TARSKI:def 1;
  hence thesis by FUNCT_1:1;
end;
