Some questions about infinitives

[liu1993918]

I have some questions about the use of infinitives.
There are some correct sentences in the MGMAT GUIDE:
1. The weight was lifted TO FREE my leg.
2. The building was demolished TO KEEP IT from falling down accidentally.

From explanation of the 1st correct sentence, I know that infinitives of purpose can be used with unnamed agents in passive-voice sentences.
However, the explanation of 2nd sentence told me that the implied subject of the infinitive TO KEEP is building and IT is needed here to refer to building.
So, I get confused. If the the 1st explanation is right, we don't need it in the 2nd sentence, right?

[RonPurewal]

"it" in sentence #2 plays the same role that "my leg" plays in sentence #1, so i guess i don't understand your question.