Need to flag redundant/conflicting "goes back N years ago"

I just heard this in a YouTube video:
> And this theory **goes back 40 years ago** to a deal that most people have probably never even heard of.

You "go back" a period/span of time. But "ago" indicates a point in time.

- And this theory **goes back 40 years** to a deal that most people have probably never even heard of.
- And this theory **goes back to 40 years ago** to a deal that most people have probably never even heard of.

Some research needs to be done to see if it also applies to any of the other inflections: go back, going back, gone back, went back.

There are already some `Expr` that can match units/periods of time. They can probably be used as-is or perhaps modified slightly if need be.

More examples:
- You have to **go back 20 years ago** to find a Prize awarded for an investigation of the Nation of Islam's questionable business dealings.
- In order to search for various errors, we need to be able to **go back hours or even days ago** in the logs.
- **Go back 20 years ago** and look at what the masses were using: instant cameras or some variant.

Locate the existing Expr implementations that match units or periods of time, then compare their handling of “go back” with the examples in this issue. Check the go back, going back, gone back, and went back inflections; done means redundant or conflicting “ago” usage is flagged without incorrectly flagging the valid alternatives.

Written by the indexing model from the issue text.