theorem Th12:
 x in proj1_3 X implies ex y,z st [x,y,z] in X
proof assume x in proj1_3 X;
  then consider y such that
A1: [x,y] in proj1 X by Def12;
  consider z such that
A2: [[x,y],z] in X by A1,Def12;
 take y,z;
 thus thesis by A2;
end;
