theorem Th20:
 x in proj2_4 X implies ex x1,x2,x3 st [x1,x,x2,x3] in X
proof assume x in proj2_4 X;
  then consider x1 such that
A1: [x1,x] in proj1_3 X by Def13;
  consider x2 such that
A2: [[x1,x],x2] in proj1 X by A1,Def12;
  consider x3 such that
A3: [[[x1,x],x2],x3] in X by A2,Def12;
 take x1,x2,x3;
 thus thesis by A3;
end;
