restart;
# Compute Truncation Error for a Method
m:=4;

T:=y(t+h)-y(t)-h*f(t+h/2,(1/2)*(y(t+h)+y(t)));

S:=series(T,h,m);

eq[1]:=D(t->y(t))(t)=f(t,y(t));

for j from 1 to m-2
do
    eq[j+1]:=simplify(subs(seq(eq[i],i=1..j),D(unapply(eq[j],t))(t)));
od;


simplify(subs(seq(eq[i],i=1..m-1),S));