theorem Th73:
  [x,y] is quasi-type of C iff
  x is finite Subset of QuasiAdjs C & y is pure expression of C, a_Type C
proof
  thus
  now
    assume [x,y] is quasi-type of C;
    then ex A,q st ( [x,y] = [A,q]) by Th72;
    hence x is finite Subset of QuasiAdjs C &
    y is pure expression of C, a_Type C by XTUPLE_0:1;
  end;
  thus thesis by Th72;
end;
