reserve A,B,C for Ordinal;
reserve a,b,c,d for natural Ordinal;
reserve l,m,n for natural Ordinal;

theorem Th27:
  a <> {} implies RED(a,a) = 1
proof
  assume
A1: a <> {};
  thus RED(a,a) = a div^ a by Th16
    .= a*^1 div^ a by ORDINAL2:39
    .= 1 by A1,ORDINAL3:68;
end;
