I start with a expression, for example where I have

`ex := \eta^{\mu \nu}\partial_{\mu}{\phi} \partial_{\nu}{\phi};`

then I do

`substitute(ex, $\partial_{\mu}{\phi} -> -i*p_{\mu}*\phi$);`

This command will change both terms $\partial_{\mu}\phi$, but I'd like to substitute just the first term, because the second term I'd like to substitute by $-i k_{\mu}$. How can I do this?