Unexpected match of structures

This structure:

```
fill(_);
if ($ifCond) {
fill(_);
}
rect(_);
```

matches true to:

```
if (mouseIsPressed) {
fill(255, 0, 0);
}
fill(0, 255, 68);
rect(0, 0, 400, 200);
```

I replicated on http://khan.github.io/structuredjs/, which I recently pushed, so it should be up to date.

Reproduce the reported match at http://khan.github.io/structuredjs/ using the two code snippets in the issue. Trace the structural-matching entry point to determine why the example is considered a match, then add or update coverage for the case and verify that the resulting behavior matches the documented expectation.

Written by the indexing model from the issue text.