reconsider x = x, y = y, z = z, t = t as set by TARSKI:1;
<%x,y,z,t%> = ((<%x%> ^ <%y%>) ^ <%z%>) ^ <%t%> by AFINSQ_1:def 14;
hence <%x,y,z,t%> is 4 -element ; :: thesis: verum