
    // The number of calls to 'expensive' should be the size of the
    // output, plus some margin from the stencil, plus a little
    // redundant recompute at the split boundary.
    int correct = (out.height() + 2 + 2) * out.width();
    if (call_count != correct) {
        printf("number of calls to producer was %d instead of %d\n",
               call_count, correct);
        return 1;
    }