Symbolic toolbox: symbolic function with a function as a "variable"

Question

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I tried to defined symbolic function with a function as a variable:

syms y(t)

syms f(t,y(t)) - but it dosn't work

and additionally

f(t,y(t))=diff(y(t))

script below is working

% LOCAL ACCURACY OF MIDPOINT METHOD
% FOR SOLVING ODE
syms f(t,z) y(t);syms h t; 
assume(diff(y(t))==f(t,y(t)))  % ODE to be solved
r(h)=y(0)-y(-h)-h*f(-h/2,y(-h)+(h/2)*f(-h,y(-h))); % error
r1=simplify(subs(diff(r,h),h,0))  % first derivative of error at t=0
r2=simplify(subs(diff(r,h,2),h,0))% second derivative of error at t=0
r3=simplify(subs(diff(r,h,3),h,0))% third derivative of error at t=0

has provided correct results (i.e. derivatives of error with respect h), but in an illegible form because the instruction "simplify" has ignored the key assumption:

assume(diff(y(t))==f(t,y(t)));

in the case of r1 and r2, the result should be 0...