title: Function of a function
body: <p>Say, I have some function <code>f[a_,b_,c_]=a+b+c</code>. Now I need to define another function,<code>g[f_,d_]=f+d</code>, where the <code>f</code> here should be replaced by the definition of <code>f</code>, resulting in <code>a+b+c+d</code>. </p>
  
  <p>How do I do this? I tried <code>g[f_,d_]=f+d/.f-&gt;Definition[f]</code>, however that didn't work.</p>