"$u_k$" above
line
{
arrow right_ del
B0: circle "$b_0$"
arrow right linewid/2
S: circle "$\sum$"
line right 2*circlerad + del + S.w.x-B0.e.x
{arrow ; "$y_k$" above }
down_
delay
{line left_ del ; circle "$-a_1$" ; arrow from last circle to S chop}
line dashed down_ del
delay
{line left_ del ; circle "$-a_n$" ; arrow from last circle to S chop}
}
down_
delay
{line right_ del ; circle "$b_1$" ; arrow from last circle to S chop}
line dashed down_ del
delay
{line right_ del ; circle "$b_n$" ; arrow from last circle to S chop}
PE
