Characteristic inner loop
DO I = 1, NEDGE
Y(EDGE1(I)) = Y(EDGE1(I)) +
& F(X(EDGE1(I)), X(EDGE2(I)))
Y(EDGE2(I)) = Y(EDGE2(I)) +
& G(X(EDGE1(I)), X(EDGE2(I)))
ENDDO
Value of Y at node is sum of terms, each depending on value of X at ends of edge connected to node.