theorem
   proj1 proj1 {[x1,x2,x3],[y1,y2,y3]} = {x1,y1}
  proof
   thus proj1 proj1 {[x1,x2,x3],[y1,y2,y3]}
       = proj1 {[x1,x2],[y1,y2]} by RELAT_1:10
      .= {x1,y1} by RELAT_1:10;
  end;
