How to determine if an integral is unevaluated?

If you want to check whether Maxima has returned an integral in closed
form, it's not enough to check if the top-level operator is
noun(integrate).

The most trivial example of this is integrate(-f(x),x), which returns
-'integrate(f(x),x).  More interesting cases are

  integrate(1/(x^7+1),x)

and

  integrate(erf(x)/x^2,x)

which return expressions involving noun-form integrals, e.g.

            2
     /   - x
     [ %E
   2 I ------ dx
     ]   x
     /             ERF(x)
   ------------- - ------
     SQRT(%PI)       x

I don't think there's a standard function that will search an expression
for a given operator, so you can either write a little recursive routine
or do something simple but inefficient like

   is( expr = subst('foobar,noun('integrate),expr) )

(untested).

      -s