function z=psi(i,tri,xy)
% Quadratic triangular basis function associeted to mid point of edge i
% Uses L, same parameters.
j=mod(i+1,3);k=mod(i+2,3);
if j==0, j=3; end; 
if k==0, k=3; end;
z=4*L(j,tri,xy).*L(k,tri,xy);
