symbolic n;
{[iw] -> [ir]  : 
	1 <= iw, ir <= 2n and iw=ir 
	and ! exists ( ik,jk : 1 <= ik <= 2n && 1 <= jk < n and
			iw <= ik = ir and 2jk = ir )
	and ! exists ( ik,jk : 1 <= ik <= 2n && 1 <= jk < n and
			iw <= ik = ir and 2jk+1 = ir )
	};