reserve X for BCI-algebra;
reserve X1 for non empty Subset of X;
reserve A,I for Ideal of X;
reserve x,y,z for Element of X;
reserve a for Element of A;

theorem
  for X being BCI-algebra,A being Ideal of X st X is quasi-associative
  holds A is closed
by BCIALG_1:71,Th5;
