4.18 Find functions whose first argument is \(z\)

restart; 
expr := 3*sin(z)+t+3*f(z,t,y)*t+g(x,t); 
res  := indets(expr, patfunc(identical(z), anything)); 
if numelems(res)<>0 then 
   print("Found these functions",res); 
else 
   print("could not find any function)"); 
fi;
 

gives

"Found these functions", f(z, t, y), sin(z)