Can you make symbolic calculations on a function defined numerically? (Matlab)

Let's say I already have a function like this

function Result = z(x, y)
Result=-x*y;

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Given that function I want to solve this ODE. (the RHS comes from that function)

>> dsolve('Dy=-x*y','y(0)=2','x')
ans =
2*exp(-x^2/2)

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However I don't want to update the second bit of code every time the first changes. I was wondering if there's a way for the result to be automatically updated.